diff --git a/src/Geo/Country.php b/src/Geo/Country.php
index 2d3af33..933e2e1 100644
--- a/src/Geo/Country.php
+++ b/src/Geo/Country.php
@@ -267,17 +267,17 @@ class Country
     /**
      * @var string
      */
-    private $isoCode;
+    protected $isoCode;
 
     /**
      * @var string
      */
-    private $name;
+    protected $name;
 
     /**
      * @param string $isoCode
      */
-    private function __construct(string $isoCode, string $name)
+    protected function __construct(string $isoCode, string $name)
     {
         $this->isoCode = $isoCode;
         $this->name = $name;
@@ -296,7 +296,7 @@ public static function __callStatic(string $name, array $arguments)
             throw new InvalidArgumentException(sprintf('Invalid country code: %s', $isoCode));
         }
 
-        return new self($isoCode, $name);
+        return new static($isoCode, $name);
     }
 
     /**
diff --git a/src/Geo/Japan/Prefecture.php b/src/Geo/Japan/Prefecture.php
index 3608d21..b2dd853 100644
--- a/src/Geo/Japan/Prefecture.php
+++ b/src/Geo/Japan/Prefecture.php
@@ -112,27 +112,27 @@ class Prefecture
     /**
      * @var string
      */
-    private $isoCode;
+    protected $isoCode;
 
     /**
      * @var string
      */
-    private $name;
+    protected $name;
 
     /**
      * @var string
      */
-    private $kanjiName;
+    protected $kanjiName;
 
     /**
      * @var PrefectureSuffix
      */
-    private $suffix;
+    protected $suffix;
 
     /**
      * @var Region
      */
-    private $region;
+    protected $region;
 
     /**
      * @param string $isoCode
@@ -141,7 +141,7 @@ class Prefecture
      * @param PrefectureSuffix $suffix
      * @param Region $region
      */
-    private function __construct(
+    protected function __construct(
         string $isoCode,
         string $name,
         string $kanjiName,
@@ -172,7 +172,7 @@ public static function __callStatic(string $name, array $arguments): self
         $suffixFactoryMethod = $datum['suffix'];
         $regionFactoryMethod = $datum['region'];
 
-        return new self(
+        return new static(
             $datum['isoCode'],
             $datum['name'],
             $datum['kanjiName'],
@@ -197,7 +197,7 @@ public static function ofKanjiName(string $kanjiName): self
         $suffixFactoryMethod = $datum['suffix'];
         $regionFactoryMethod = $datum['region'];
 
-        return new self (
+        return new static(
             $datum['isoCode'],
             $datum['name'],
             $datum['kanjiName'],
@@ -229,7 +229,7 @@ public static function ofSuffixedKanjiName(string $suffixedKanjiName): self
         $suffixFactoryMethod = $datum['suffix'];
         $regionFactoryMethod = $datum['region'];
 
-        return new self (
+        return new static(
             $datum['isoCode'],
             $datum['name'],
             $datum['kanjiName'],
diff --git a/src/Geo/Japan/PrefectureSuffix.php b/src/Geo/Japan/PrefectureSuffix.php
index 0e10fc5..0254f55 100644
--- a/src/Geo/Japan/PrefectureSuffix.php
+++ b/src/Geo/Japan/PrefectureSuffix.php
@@ -25,18 +25,18 @@ class PrefectureSuffix
     /**
      * @var string
      */
-    private $name;
+    protected $name;
 
     /**
      * @var string
      */
-    private $kanjiName;
+    protected $kanjiName;
 
     /**
      * @param string $name
      * @param string $kanjiName
      */
-    private function __construct(string $name, string $kanjiName)
+    protected function __construct(string $name, string $kanjiName)
     {
         $this->name = $name;
         $this->kanjiName = $kanjiName;
@@ -57,7 +57,7 @@ public static function __callStatic(string $name, array $arguments): self
         }
         $datum = array_shift($filteredData);
 
-        return new self($datum['name'], $datum['kanjiName']);
+        return new static($datum['name'], $datum['kanjiName']);
     }
 
     /**
diff --git a/src/Geo/Japan/Region.php b/src/Geo/Japan/Region.php
index a22f3e0..30f70fa 100644
--- a/src/Geo/Japan/Region.php
+++ b/src/Geo/Japan/Region.php
@@ -37,17 +37,18 @@ class Region
     /**
      * @var string
      */
-    private $name;
+    protected $name;
 
     /**
      * @var string
      */
-    private $kanjiName;
+    protected $kanjiName;
 
     /**
-     * @param array $region
+     * @param string $name
+     * @param string $kanjiName
      */
-    private function __construct(string $name, string $kanjiName)
+    protected function __construct(string $name, string $kanjiName)
     {
         $this->name = $name;
         $this->kanjiName = $kanjiName;
@@ -81,7 +82,7 @@ public static function fromName(string $name): self
             new InvalidArgumentException(sprintf('Invalid region name: %s', $name))
         );
 
-        return new self($datum['name'], $datum['kanjiName']);
+        return new static($datum['name'], $datum['kanjiName']);
     }
 
     /**
@@ -96,7 +97,7 @@ public static function fromKanjiName(string $kanjiName): self
             new InvalidArgumentException(sprintf('Invalid region name in kanji: %s', $kanjiName))
         );
 
-        return new self($datum['name'], $datum['kanjiName']);
+        return new static($datum['name'], $datum['kanjiName']);
     }
 
     /**
@@ -131,7 +132,7 @@ public function equals(self $other): bool
      * @return array
      * @throws Exception
      */
-    private static function filterData(string $key, string $value, Exception $e): array
+    protected static function filterData(string $key, string $value, Exception $e): array
     {
         $filteredData = array_filter(self::DATA, function (array $datum) use ($key, $value) {
             return $value === $datum[$key];
@@ -147,8 +148,8 @@ private static function filterData(string $key, string $value, Exception $e): ar
      * @param array $datum
      * @return self
      */
-    private static function createInstance(array $datum): self
+    protected static function createInstance(array $datum): self
     {
-        return new self($datum['name'], $datum['kanjiName']);
+        return new static($datum['name'], $datum['kanjiName']);
     }
 }
diff --git a/src/Geo/Latitude.php b/src/Geo/Latitude.php
index 33424e4..59493c0 100644
--- a/src/Geo/Latitude.php
+++ b/src/Geo/Latitude.php
@@ -9,7 +9,7 @@ class Latitude
     /**
      * @var float
      */
-    private $latitude;
+    protected $latitude;
 
     /**
      * @param float $latitude
diff --git a/src/Geo/Longitude.php b/src/Geo/Longitude.php
index e1b9672..72b49ed 100644
--- a/src/Geo/Longitude.php
+++ b/src/Geo/Longitude.php
@@ -9,7 +9,7 @@ class Longitude
     /**
      * @var float
      */
-    private $longitude;
+    protected $longitude;
 
     /**
      * @param float $longitude
diff --git a/src/Geo/Position.php b/src/Geo/Position.php
index 64753f5..b0e05d3 100644
--- a/src/Geo/Position.php
+++ b/src/Geo/Position.php
@@ -7,12 +7,12 @@ class Position
     /**
      * @var Latitude
      */
-    private $latitude;
+    protected $latitude;
 
     /**
      * @var Longitude
      */
-    private $longitude;
+    protected $longitude;
 
     /**
      * @param Latitude $latitude
diff --git a/src/Money/Currency.php b/src/Money/Currency.php
index 9d19228..48e213c 100644
--- a/src/Money/Currency.php
+++ b/src/Money/Currency.php
@@ -179,17 +179,17 @@ class Currency
     /**
      * @var string
      */
-    private $isoCode;
+    protected $isoCode;
 
     /**
      * @var int
      */
-    private $decimalPlaces;
+    protected $decimalPlaces;
 
     /**
      * @param string $isoCode
      */
-    private function __construct(string $isoCode)
+    protected function __construct(string $isoCode)
     {
         $this->isoCode = $isoCode;
         if (in_array($isoCode, ['BHD', 'IQD', 'JOD', 'KWD', 'LYD', 'OMR', 'TND'])) {
@@ -210,9 +210,9 @@ private function __construct(string $isoCode)
     /**
      * @param string $name
      * @param array $arguments
-     * @return self
+     * @return Currency
      */
-    public static function __callStatic(string $name, array $arguments)
+    public static function __callStatic(string $name, array $arguments): Currency
     {
         $isoCode = strtoupper($name);
         $name = Intl::getCurrencyBundle()->getCurrencyName($isoCode);
@@ -220,7 +220,7 @@ public static function __callStatic(string $name, array $arguments)
             throw new InvalidArgumentException(sprintf('Invalid currency code: %s', $isoCode));
         }
 
-        return new self($isoCode);
+        return new static($isoCode);
     }
 
     /**
diff --git a/src/Money/Money.php b/src/Money/Money.php
index f32dda7..c234bd8 100644
--- a/src/Money/Money.php
+++ b/src/Money/Money.php
@@ -9,12 +9,12 @@ class Money
     /**
      * @var int
      */
-    private $amount;
+    protected $amount;
 
     /**
      * @var Currency
      */
-    private $currency;
+    protected $currency;
 
     /**
      * @param int $amount
@@ -52,18 +52,18 @@ public function equals(self $other): bool
     }
 
     /**
-     * @return self
+     * @return Money
      */
-    public function duplicate(): self
+    public function duplicate()
     {
-        return new self($this->amount(), $this->currency());
+        return new static($this->amount(), $this->currency());
     }
 
     /**
      * @param self $other
-     * @return self
+     * @return Money
      */
-    public function add(self $other): self
+    public function add(self $other): Money
     {
         if (!$this->currency()->equals($other->currency())) {
             throw new InvalidArgumentException(
@@ -75,7 +75,7 @@ public function add(self $other): self
             );
         }
 
-        return new self($this->amount() + $other->amount(), $this->currency());
+        return new static($this->amount() + $other->amount(), $this->currency());
     }
 
     /**
@@ -94,7 +94,7 @@ public function sub(self $other): self
             );
         }
 
-        return new self($this->amount() - $other->amount(), $this->currency());
+        return new static($this->amount() - $other->amount(), $this->currency());
     }
 
     /**
@@ -103,7 +103,7 @@ public function sub(self $other): self
      */
     public function multiply(float $multiplier): self
     {
-        return new self(intval(floor($this->amount() * $multiplier)), $this->currency());
+        return new static(intval(floor($this->amount() * $multiplier)), $this->currency());
     }
 
     /**
@@ -121,7 +121,7 @@ public function increaseAmountBy(int $amount): self
             );
         }
 
-        return new self($this->amount() + $amount, $this->currency());
+        return new static($this->amount() + $amount, $this->currency());
     }
 
     /**
@@ -139,6 +139,6 @@ public function decreaseAmountBy(int $amount): self
             );
         }
 
-        return new self($this->amount() - $amount, $this->currency());
+        return new static($this->amount() - $amount, $this->currency());
     }
 }
diff --git a/src/Time/Date.php b/src/Time/Date.php
index 88dffb1..a25de6d 100644
--- a/src/Time/Date.php
+++ b/src/Time/Date.php
@@ -12,17 +12,17 @@ class Date
     /**
      * @var int
      */
-    private $year;
+    protected $year;
 
     /**
      * @var int
      */
-    private $month;
+    protected $month;
 
     /**
      * @var int
      */
-    private $day;
+    protected $day;
 
     /**
      * @param int $year
diff --git a/src/Time/TimeLength.php b/src/Time/TimeLength.php
index c72a50b..756ec4b 100644
--- a/src/Time/TimeLength.php
+++ b/src/Time/TimeLength.php
@@ -9,12 +9,12 @@ class TimeLength
     /**
      * @var int
      */
-    private $lengthInSeconds;
+    protected $lengthInSeconds;
 
     /**
      * @param int $lengthInSeconds
      */
-    private function __construct(int $lengthInSeconds)
+    protected function __construct(int $lengthInSeconds)
     {
         if (!($lengthInSeconds >= 0)) {
             throw new InvalidArgumentException(sprintf('Length cannot be negative in seconds: %s', $lengthInSeconds));
@@ -25,38 +25,38 @@ private function __construct(int $lengthInSeconds)
 
     /**
      * @param int $seconds
-     * @return self
+     * @return TimeLength
      */
-    public static function ofSeconds(int $seconds): self
+    public static function ofSeconds(int $seconds): TimeLength
     {
-        return new self($seconds);
+        return new static($seconds);
     }
 
     /**
      * @param int $minutes
-     * @return self
+     * @return TimeLength
      */
-    public static function ofMinutes(int $minutes): self
+    public static function ofMinutes(int $minutes): TimeLength
     {
-        return new self($minutes * 60);
+        return new static($minutes * 60);
     }
 
     /**
      * @param int $hours
-     * @return self
+     * @return TimeLength
      */
-    public static function ofHours(int $hours): self
+    public static function ofHours(int $hours): TimeLength
     {
-        return new self($hours * 3600);
+        return new static($hours * 3600);
     }
 
     /**
      * @param int $days
-     * @return self
+     * @return TimeLength
      */
-    public static function ofDays(int $days): self
+    public static function ofDays(int $days): TimeLength
     {
-        return new self($days * 86400);
+        return new static($days * 86400);
     }
 
     /**
diff --git a/src/Time/TimeOfDay.php b/src/Time/TimeOfDay.php
index d217e33..177553c 100644
--- a/src/Time/TimeOfDay.php
+++ b/src/Time/TimeOfDay.php
@@ -9,17 +9,17 @@ class TimeOfDay
     /**
      * @var int
      */
-    private $hours;
+    protected $hours;
 
     /**
      * @var int
      */
-    private $minutes;
+    protected $minutes;
 
     /**
      * @var int
      */
-    private $seconds;
+    protected $seconds;
 
     /**
      * @param int $hours
diff --git a/src/Time/TimeRange.php b/src/Time/TimeRange.php
index f1813bf..a71bcd7 100644
--- a/src/Time/TimeRange.php
+++ b/src/Time/TimeRange.php
@@ -10,12 +10,12 @@ class TimeRange
     /**
      * @var DateTimeImmutable
      */
-    private $start;
+    protected $start;
 
     /**
      * @var DateTimeImmutable
      */
-    private $end;
+    protected $end;
 
     /**
      * @param DateTimeImmutable $start
diff --git a/src/Time/TimeRangeOfDay.php b/src/Time/TimeRangeOfDay.php
index 1e561e5..c381861 100644
--- a/src/Time/TimeRangeOfDay.php
+++ b/src/Time/TimeRangeOfDay.php
@@ -9,12 +9,12 @@ class TimeRangeOfDay
     /**
      * @var TimeOfDay
      */
-    private $start;
+    protected $start;
 
     /**
      * @var TimeOfDay
      */
-    private $end;
+    protected $end;
 
     /**
      * @param TimeOfDay $start
@@ -94,7 +94,7 @@ public function overlapping(self $other): self
         $start = $this->start()->isAfter($other->start()) ? $this->start() : $other->start();
         $end = $this->end()->isBefore($other->end()) ? $this->end() : $other->end();
 
-        return new self($start, $end);
+        return new static($start, $end);
     }
 
     /**
@@ -102,7 +102,7 @@ public function overlapping(self $other): self
      */
     public static function am(): self
     {
-        return new self(new TimeOfDay(0), new TimeOfDay(12));
+        return new static(new TimeOfDay(0), new TimeOfDay(12));
     }
 
     /**
@@ -110,7 +110,7 @@ public static function am(): self
      */
     public static function pm(): self
     {
-        return new self(new TimeOfDay(12), new TimeOfDay(23, 59, 0));
+        return new static(new TimeOfDay(12), new TimeOfDay(23, 59, 0));
     }
 
     /**
@@ -118,6 +118,6 @@ public static function pm(): self
      */
     public static function allDay(): self
     {
-        return new self(new TimeOfDay(0), new TimeOfDay(23, 59, 59));
+        return new static(new TimeOfDay(0), new TimeOfDay(23, 59, 59));
     }
 }
diff --git a/src/Web/Email.php b/src/Web/Email.php
index 14a21c7..3fe2c3a 100644
--- a/src/Web/Email.php
+++ b/src/Web/Email.php
@@ -46,7 +46,7 @@ class Email
      * @param EmailAddress[] $tos
      * @param EmailAddress[] $ccs
      * @param EmailAddress[] $bccs
-     * @param EmailAttachment $attachments
+     * @param EmailAttachment[] $attachments
      */
     public function __construct(
         string $subject,
diff --git a/src/Web/EmailAddress.php b/src/Web/EmailAddress.php
index 46f55a2..4bdd7a4 100644
--- a/src/Web/EmailAddress.php
+++ b/src/Web/EmailAddress.php
@@ -9,7 +9,7 @@ class EmailAddress
     /**
      * @var string
      */
-    private $emailAddress;
+    protected $emailAddress;
 
     /**
      * @param string $emailAddress
@@ -46,7 +46,7 @@ public function emailAddress(): string
      * @param string $emailAddress
      * @return bool
      */
-    private function validateSoft(string $emailAddress): bool
+    protected function validateSoft(string $emailAddress): bool
     {
         return preg_match('/^.+\@\S+\.\S+$/', $emailAddress);
     }
@@ -55,7 +55,7 @@ private function validateSoft(string $emailAddress): bool
      * @param string $emailAddress
      * @return bool
      */
-    private function validateStrict(string $emailAddress): bool
+    protected function validateStrict(string $emailAddress): bool
     {
         return filter_var($emailAddress, FILTER_VALIDATE_EMAIL);
     }
diff --git a/src/Web/EmailAttachment.php b/src/Web/EmailAttachment.php
index fce662a..9544b58 100644
--- a/src/Web/EmailAttachment.php
+++ b/src/Web/EmailAttachment.php
@@ -153,21 +153,21 @@ class EmailAttachment
     /**
      * @var string
      */
-    private $content;
+    protected $content;
 
     /**
      * @var string
      */
-    private $fileName;
+    protected $fileName;
 
     /**
      * @var string
      */
-    private $mimeType;
+    protected $mimeType;
 
     /**
      * @param string $content
-     * @param string $filename
+     * @param string $fileName
      */
     public function __construct(string $content, string $fileName)
     {
diff --git a/src/Web/HttpMethod.php b/src/Web/HttpMethod.php
index de86683..9a4338b 100644
--- a/src/Web/HttpMethod.php
+++ b/src/Web/HttpMethod.php
@@ -52,12 +52,12 @@ class HttpMethod
     /**
      * @var string
      */
-    private $httpMethod;
+    protected $httpMethod;
 
     /**
      * @param string $httpMethod
      */
-    private function __construct(string $httpMethod)
+    protected function __construct(string $httpMethod)
     {
         $this->httpMethod = $httpMethod;
     }
@@ -67,7 +67,7 @@ private function __construct(string $httpMethod)
      */
     public static function get(): self
     {
-        return new self(self::GET);
+        return new static(self::GET);
     }
 
     /**
@@ -75,7 +75,7 @@ public static function get(): self
      */
     public static function head(): self
     {
-        return new self(self::HEAD);
+        return new static(self::HEAD);
     }
 
     /**
@@ -83,7 +83,7 @@ public static function head(): self
      */
     public static function post(): self
     {
-        return new self(self::POST);
+        return new static(self::POST);
     }
 
     /**
@@ -91,7 +91,7 @@ public static function post(): self
      */
     public static function put(): self
     {
-        return new self(self::PUT);
+        return new static(self::PUT);
     }
 
     /**
@@ -99,7 +99,7 @@ public static function put(): self
      */
     public static function delete(): self
     {
-        return new self(self::DELETE);
+        return new static(self::DELETE);
     }
 
     /**
@@ -107,7 +107,7 @@ public static function delete(): self
      */
     public static function connect(): self
     {
-        return new self(self::CONNECT);
+        return new static(self::CONNECT);
     }
 
     /**
@@ -115,7 +115,7 @@ public static function connect(): self
      */
     public static function options(): self
     {
-        return new self(self::OPTIONS);
+        return new static(self::OPTIONS);
     }
 
     /**
@@ -123,7 +123,7 @@ public static function options(): self
      */
     public static function trace(): self
     {
-        return new self(self::TRACE);
+        return new static(self::TRACE);
     }
 
     /**
@@ -131,7 +131,7 @@ public static function trace(): self
      */
     public static function patch(): self
     {
-        return new self(self::PATCH);
+        return new static(self::PATCH);
     }
 
     /**
diff --git a/src/Web/IpAddressV4.php b/src/Web/IpAddressV4.php
index b9f92bb..7b2dea0 100644
--- a/src/Web/IpAddressV4.php
+++ b/src/Web/IpAddressV4.php
@@ -9,7 +9,7 @@ class IpAddressV4
     /**
      * @var string
      */
-    private $ipAddressV4;
+    protected $ipAddressV4;
 
     /**
      * @param string $ipAddressV4
diff --git a/src/Web/IpAddressV6.php b/src/Web/IpAddressV6.php
index 6575250..7fa8859 100644
--- a/src/Web/IpAddressV6.php
+++ b/src/Web/IpAddressV6.php
@@ -9,7 +9,7 @@ class IpAddressV6
     /**
      * @var string
      */
-    private $ipAddressV6;
+    protected $ipAddressV6;
 
     /**
      * @param string $ipAddressV6
diff --git a/src/Web/Url.php b/src/Web/Url.php
index c84aec7..7111632 100644
--- a/src/Web/Url.php
+++ b/src/Web/Url.php
@@ -9,7 +9,7 @@ class Url
     /**
      * @var string
      */
-    private $url;
+    protected $url;
 
     /**
      * @param string $url