diff --git a/tests/Currencies/CachedCurrenciesTest.php b/tests/Currencies/CachedCurrenciesTest.php
index 38ce1de7..0993e817 100644
--- a/tests/Currencies/CachedCurrenciesTest.php
+++ b/tests/Currencies/CachedCurrenciesTest.php
@@ -122,4 +122,71 @@ public function it_is_iterable(): void
             iterator_to_array(new CachedCurrencies($wrappedCurrencies, $cache))
         );
     }
+
+    /** @test */
+    public function it_checks_subunits_from_the_cache(): void
+    {
+        $currency = new Currency('EUR');
+
+        $hit               = $this->createMock(CacheItemInterface::class);
+        $cache             = $this->createMock(CacheItemPoolInterface::class);
+        $wrappedCurrencies = $this->createMock(Currencies::class);
+
+        $hit->method('isHit')
+            ->willReturn(true);
+        $hit->expects(self::never())
+            ->method('set');
+        $hit->method('get')
+            ->willReturn(2);
+
+        $cache->method('getItem')
+            ->with('currency|subunit|EUR')
+            ->willReturn($hit);
+        $cache->expects(self::never())
+            ->method('save');
+
+        $wrappedCurrencies->expects(self::never())
+            ->method('subunitFor');
+
+        self::assertEquals(
+            2,
+            (new CachedCurrencies($wrappedCurrencies, $cache))
+                ->subunitFor($currency)
+        );
+    }
+
+    /** @test */
+    public function it_saves_subunits_to_the_cache(): void
+    {
+        $currency = new Currency('EUR');
+
+        $hit               = $this->createMock(CacheItemInterface::class);
+        $cache             = $this->createMock(CacheItemPoolInterface::class);
+        $wrappedCurrencies = $this->createMock(Currencies::class);
+
+        $hit->method('isHit')
+            ->willReturn(false);
+        $hit->expects(self::once())
+            ->method('set')
+            ->with(2);
+        $hit->expects(self::once())
+            ->method('get')
+            ->willReturn(2);
+
+        $cache->method('getItem')
+            ->with('currency|subunit|EUR')
+            ->willReturn($hit);
+        $cache->expects(self::once())
+            ->method('save');
+
+        $wrappedCurrencies->expects(self::once())
+            ->method('subunitFor')
+            ->willReturn(2);
+
+        self::assertEquals(
+            2,
+            (new CachedCurrencies($wrappedCurrencies, $cache))
+                ->subunitFor($currency)
+        );
+    }
 }
diff --git a/tests/CurrencyPairTest.php b/tests/CurrencyPairTest.php
index bba68ce9..7ccdfebe 100644
--- a/tests/CurrencyPairTest.php
+++ b/tests/CurrencyPairTest.php
@@ -16,54 +16,64 @@ final class CurrencyPairTest extends TestCase
     /**
      * @test
      */
-    public function itConvertsToJson(): void
-    {
-        $expectedJson = '{"baseCurrency":"EUR","counterCurrency":"USD","ratio":"1.25"}';
-        $actualJson   = json_encode(new CurrencyPair(new Currency('EUR'), new Currency('USD'), '1.25'));
-
-        self::assertEquals($expectedJson, $actualJson);
-    }
-
-    /** @test */
-    public function it_parses_an_iso_string(): void
+    public function itProvidesGetters(): void
     {
-        self::assertEquals(
-            new CurrencyPair(new Currency('EUR'), new Currency('USD'), '1.250000'),
-            CurrencyPair::createFromIso('EUR/USD 1.250000')
+        $pair = new CurrencyPair(
+            new Currency('USD'),
+            new Currency('EUR'),
+            '1.0'
         );
+
+        self::assertEquals('USD', $pair->getBaseCurrency()->getCode());
+        self::assertEquals('EUR', $pair->getCounterCurrency()->getCode());
+        self::assertEquals('1.0', $pair->getConversionRatio());
     }
 
-    /** @test */
-    public function it_equals_to_another_currency_pair(): void
+    /**
+     * @test
+     */
+    public function itProvidesEquality(): void
     {
-        $pair = new CurrencyPair(
-            new Currency('EUR'),
+        $pair1 = new CurrencyPair(
             new Currency('USD'),
-            '1.250000'
+            new Currency('EUR'),
+            '1.0'
         );
 
-        self::assertFalse($pair->equals(new CurrencyPair(
-            new Currency('GBP'),
+        self::assertTrue($pair1->equals(new CurrencyPair(
             new Currency('USD'),
-            '1.250000'
-        )));
-
-        self::assertFalse($pair->equals(new CurrencyPair(
             new Currency('EUR'),
-            new Currency('GBP'),
-            '1.250000'
+            '1.0'
         )));
-
-        self::assertFalse($pair->equals(new CurrencyPair(
-            new Currency('EUR'),
+        self::assertFalse($pair1->equals(new CurrencyPair(
             new Currency('USD'),
-            '1.5000'
+            new Currency('EUR'),
+            '2.0'
         )));
+    }
 
-        self::assertTrue($pair->equals(new CurrencyPair(
-            new Currency('EUR'),
+    /**
+     * @test
+     */
+    public function itConvertsToJson(): void
+    {
+        $pair = new CurrencyPair(
             new Currency('USD'),
-            '1.250000'
-        )));
+            new Currency('EUR'),
+            '1.0'
+        );
+
+        self::assertEquals('{"baseCurrency":"USD","counterCurrency":"EUR","ratio":"1.0"}', json_encode($pair));
+    }
+
+    /**
+     * @test
+     */
+    public function itCanBeCreatedWithAnIsoString(): void
+    {
+        $pair = CurrencyPair::createFromIso('EUR/USD 1.2500');
+        self::assertEquals('EUR', $pair->getBaseCurrency()->getCode());
+        self::assertEquals('USD', $pair->getCounterCurrency()->getCode());
+        self::assertEquals('1.2500', $pair->getConversionRatio());
     }
 }
diff --git a/tests/CurrencyTest.php b/tests/CurrencyTest.php
index 8a07a018..2500991a 100644
--- a/tests/CurrencyTest.php
+++ b/tests/CurrencyTest.php
@@ -27,4 +27,21 @@ public function itAppliesUppercase(): void
     {
         self::assertEquals('USD', (new Currency('usd'))->getCode());
     }
+
+    /**
+     * @test
+     */
+    public function itIsStringable(): void
+    {
+        self::assertEquals('USD', (string) new Currency('usd'));
+    }
+
+    /**
+     * @test
+     */
+    public function itProvidesEqualityComparison(): void
+    {
+        $currency = new Currency('usd');
+        self::assertTrue($currency->equals(new Currency('USD')));
+    }
 }
diff --git a/tests/Exchange/FixedExchangeTest.php b/tests/Exchange/FixedExchangeTest.php
new file mode 100644
index 00000000..b26891a6
--- /dev/null
+++ b/tests/Exchange/FixedExchangeTest.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Exchange;
+
+use Money\Currency;
+use Money\Exception\UnresolvableCurrencyPairException;
+use Money\Exchange\FixedExchange;
+use PHPUnit\Framework\TestCase;
+
+/** @covers \Money\Exchange\FixedExchange */
+final class FixedExchangeTest extends TestCase
+{
+    /** @test */
+    public function it_exchanges_currencies(): void
+    {
+        $exchange     = new FixedExchange(['EUR' => ['USD' => '1.2500']]);
+        $currencyPair = $exchange->quote(new Currency('EUR'), new Currency('USD'));
+
+        self::assertEquals('EUR', $currencyPair->getBaseCurrency());
+        self::assertEquals('USD', $currencyPair->getCounterCurrency());
+        self::assertEquals('1.2500', $currencyPair->getConversionRatio());
+    }
+
+    /** @test */
+    public function it_throws_when_quoting_unknown_currency(): void
+    {
+        $this->expectException(UnresolvableCurrencyPairException::class);
+        $exchange = new FixedExchange(['EUR' => ['USD' => '1.2500']]);
+        $exchange->quote(new Currency('EUR'), new Currency('SOME'));
+    }
+}
diff --git a/tests/MoneyTest.php b/tests/MoneyTest.php
index 1aa23d35..5c20ab94 100644
--- a/tests/MoneyTest.php
+++ b/tests/MoneyTest.php
@@ -400,6 +400,26 @@ public function itRoundsToUnit($amount, $unit, $expected, $roundingMode): void
         self::assertEquals(Money::EUR($expected), Money::EUR($amount)->roundToUnit($unit, $roundingMode));
     }
 
+    /** @test */
+    public function itThrowsWithDecimal(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        new Money('5.1', new Currency(self::CURRENCY));
+    }
+
+    /**
+     * @test
+     */
+    public function itThrowsWhenComparingDifferentCurrencies(): void
+    {
+        $money = new Money('5', new Currency(self::CURRENCY));
+
+        $this->expectException(InvalidArgumentException::class);
+
+        /** @psalm-suppress UnusedMethodCall */
+        $money->compare(new Money('5', new Currency('SOME')));
+    }
+
     /**
      * @psalm-return non-empty-list<array{
      *     int|numeric-string,