diff --git a/tests/Unit/RuleSet/RuleSetTest.php b/tests/Unit/RuleSet/RuleSetTest.php
index 847078bb..303e718b 100644
--- a/tests/Unit/RuleSet/RuleSetTest.php
+++ b/tests/Unit/RuleSet/RuleSetTest.php
@@ -9,6 +9,7 @@
 use Sabberworm\CSS\Rule\Rule;
 use Sabberworm\CSS\RuleSet\RuleSet;
 use Sabberworm\CSS\Tests\Unit\RuleSet\Fixtures\ConcreteRuleSet;
+use TRegx\DataProvider\DataProviders;
 
 /**
  * @covers \Sabberworm\CSS\RuleSet\RuleSet
@@ -27,6 +28,148 @@ public function implementsCSSElement()
         self::assertInstanceOf(CSSElement::class, $subject);
     }
 
+    /**
+     * @return array<string, array{0: list<non-empty-string>}>
+     */
+    public static function providePropertyNamesToBeSetInitially()
+    {
+        return [
+            'no properties' => [[]],
+            'one property' => [['color']],
+            'two different properties' => [['color', 'display']],
+            'two of the same property' => [['color', 'color']],
+        ];
+    }
+
+    /**
+     * @return array<string, array{0: non-empty-string}>
+     */
+    public static function providePropertyNameToAdd()
+    {
+        return [
+            'property name `color` maybe matching that of existing declaration' => ['color'],
+            'property name `display` maybe matching that of existing declaration' => ['display'],
+            'property name `width` not matching that of existing declaration' => ['width'],
+        ];
+    }
+
+    /**
+     * @return array<string, array{0: list<string>, 1: string}>
+     */
+    public static function provideInitialPropertyNamesAndPropertyNameToAdd()
+    {
+        if (!\class_exists(DataProviders::class)) {
+            self::markTestSkipped('`DataProviders` class is not available');
+            return [];
+        }
+
+        return DataProviders::cross(self::providePropertyNamesToBeSetInitially(), self::providePropertyNameToAdd());
+    }
+
+    /**
+     * @test
+     *
+     * @param list<string> $initialPropertyNames
+     *
+     * @dataProvider provideInitialPropertyNamesAndPropertyNameToAdd
+     */
+    public function addRuleWithoutSiblingAddsRuleAfterInitialRulesAndSetsValidLineAndColumnNumbers(
+        array $initialPropertyNames,
+        string $propertyNameToAdd
+    ) {
+        if ($initialPropertyNames === []) {
+            self::markTestSkipped('currently broken - first rule added does not have valid line number set');
+        }
+
+        $subject = new ConcreteRuleSet();
+        $ruleToAdd = new Rule($propertyNameToAdd);
+        self::setRulesFromPropertyNames($subject, $initialPropertyNames);
+
+        $subject->addRule($ruleToAdd);
+
+        $rules = $subject->getRules();
+        self::assertSame($ruleToAdd, \end($rules));
+        self::assertInternalType('int', $ruleToAdd->getLineNumber(), 'line number not set');
+        self::assertGreaterThanOrEqual(1, $ruleToAdd->getLineNumber(), 'line number not valid');
+        self::assertInternalType('int', $ruleToAdd->getColumnNumber(), 'column number not set');
+        self::assertGreaterThanOrEqual(0, $ruleToAdd->getColumnNumber(), 'column number not valid');
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideInitialPropertyNamesAndPropertyNameToAdd
+     *
+     * @param list<string> $initialPropertyNames
+     */
+    public function addRuleWithOnlyLineNumberAddsRuleAndSetsColumnNumberPreservingLineNumber(
+        array $initialPropertyNames,
+        string $propertyNameToAdd
+    ) {
+        self::markTestSkipped('currently broken - does not set column number');
+
+        $subject = new ConcreteRuleSet();
+        $ruleToAdd = new Rule($propertyNameToAdd);
+        $ruleToAdd->setPosition(42);
+        self::setRulesFromPropertyNames($subject, $initialPropertyNames);
+
+        $subject->addRule($ruleToAdd);
+
+        self::assertContains($ruleToAdd, $subject->getRules());
+        self::assertInternalType('int', $ruleToAdd->getColumnNumber(), 'column number not set');
+        self::assertGreaterThanOrEqual(0, $ruleToAdd->getColumnNumber(), 'column number not valid');
+        self::assertSame(42, $ruleToAdd->getLineNumber(), 'line number not preserved');
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideInitialPropertyNamesAndPropertyNameToAdd
+     *
+     * @param list<string> $initialPropertyNames
+     */
+    public function addRuleWithOnlyColumnNumberAddsRuleAndSetsLineNumberPreservingColumnNumber(
+        array $initialPropertyNames,
+        string $propertyNameToAdd
+    ) {
+        self::markTestSkipped('currently broken - does not preserve column number');
+
+        $subject = new ConcreteRuleSet();
+        $ruleToAdd = new Rule($propertyNameToAdd);
+        $ruleToAdd->setPosition(null, 42);
+        self::setRulesFromPropertyNames($subject, $initialPropertyNames);
+
+        $subject->addRule($ruleToAdd);
+
+        self::assertContains($ruleToAdd, $subject->getRules());
+        self::assertInternalType('int', $ruleToAdd->getLineNumber(), 'line number not set');
+        self::assertGreaterThanOrEqual(1, $ruleToAdd->getLineNumber(), 'line number not valid');
+        self::assertSame(42, $ruleToAdd->getColumnNumber(), 'column number not preserved');
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideInitialPropertyNamesAndPropertyNameToAdd
+     *
+     * @param list<string> $initialPropertyNames
+     */
+    public function addRuleWithCompletePositionAddsRuleAndPreservesPosition(
+        array $initialPropertyNames,
+        string $propertyNameToAdd
+    ) {
+        $subject = new ConcreteRuleSet();
+        $ruleToAdd = new Rule($propertyNameToAdd);
+        $ruleToAdd->setPosition(42, 64);
+        self::setRulesFromPropertyNames($subject, $initialPropertyNames);
+
+        $subject->addRule($ruleToAdd);
+
+        self::assertContains($ruleToAdd, $subject->getRules());
+        self::assertSame(42, $ruleToAdd->getLineNumber(), 'line number not preserved');
+        self::assertSame(64, $ruleToAdd->getColumnNumber(), 'column number not preserved');
+    }
+
     /**
      * @return array<string, array{0: list<string>, 1: string, 2: list<string>}>
      */
@@ -171,25 +314,12 @@ public function removeMatchingRulesRemovesRulesByPropertyNamePrefixAndKeepsOther
         }
     }
 
-    /**
-     * @return array<string, array{0: list<string>}>
-     */
-    public static function providePropertyNamesToRemove()
-    {
-        return [
-            'no properties' => [[]],
-            'one property' => [['color']],
-            'two different properties' => [['color', 'display']],
-            'two of the same property' => [['color', 'color']],
-        ];
-    }
-
     /**
      * @test
      *
      * @param list<string> $propertyNamesToRemove
      *
-     * @dataProvider providePropertyNamesToRemove
+     * @dataProvider providePropertyNamesToBeSetInitially
      */
     public function removeAllRulesRemovesAllRules(array $propertyNamesToRemove)
     {