Index: Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/MacroStabilityInwardsInputValidator.cs
===================================================================
diff -u -r9655570a15a7fff650337906cb33de8042312c29 -r0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f
--- Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/MacroStabilityInwardsInputValidator.cs	(.../MacroStabilityInwardsInputValidator.cs)	(revision 9655570a15a7fff650337906cb33de8042312c29)
+++ Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/MacroStabilityInwardsInputValidator.cs	(.../MacroStabilityInwardsInputValidator.cs)	(revision 0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f)
@@ -64,6 +64,7 @@
             {
                 validationResults.AddRange(ValidateSoilLayers(inputParameters));
                 validationResults.AddRange(ValidateZoneBoundaries(inputParameters));
+                validationResults.AddRange(ValidateTangentLines(inputParameters));
             }
 
             return validationResults;
@@ -116,6 +117,15 @@
             }
         }
 
+        private static IEnumerable<string> ValidateTangentLines(MacroStabilityInwardsInput inputParameters)
+        {
+            if (inputParameters.TangentLineZTop == inputParameters.TangentLineZBottom
+                && inputParameters.TangentLineNumber != 1)
+            {
+                yield return Resources.MacroStabilityInwardsInputValidator_ValidateTangentLines_TangentLineNumber_must_be_one_when_TangentLineTop_equals_TangentLineBottom;
+            }
+        }
+
         private static bool ValidateTopOfProfileExceedsSurfaceLineTop(IMacroStabilityInwardsWaternetInput inputParameters,
                                                                       MacroStabilityInwardsSoilProfile1D soilProfile1D)
         {
Index: Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.Designer.cs
===================================================================
diff -u -r9655570a15a7fff650337906cb33de8042312c29 -r0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f
--- Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.Designer.cs	(.../Resources.Designer.cs)	(revision 9655570a15a7fff650337906cb33de8042312c29)
+++ Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.Designer.cs	(.../Resources.Designer.cs)	(revision 0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f)
@@ -171,6 +171,16 @@
         }
         
         /// <summary>
+        ///   Looks up a localized string similar to Het aantal tangentlijnen moet 1 zijn wanneer tangentlijn Z-boven gelijk is aan tangentlijn Z-onder..
+        /// </summary>
+        internal static string MacroStabilityInwardsInputValidator_ValidateTangentLines_TangentLineNumber_must_be_one_when_TangentLineTop_equals_TangentLineBottom {
+            get {
+                return ResourceManager.GetString("MacroStabilityInwardsInputValidator_ValidateTangentLines_TangentLineNumber_must_b" +
+                        "e_one_when_TangentLineTop_equals_TangentLineBottom", resourceCulture);
+            }
+        }
+        
+        /// <summary>
         ///   Looks up a localized string similar to Zoneringsgrens links moet kleiner zijn dan of gelijk zijn aan zoneringsgrens rechts..
         /// </summary>
         internal static string MacroStabilityInwardsInputValidator_ValidateZoneBoundaries_ZoneBoundaries_BoundaryLeft_should_be_smaller_than_or_equal_to_BoundaryRight {
Index: Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.resx
===================================================================
diff -u -r9655570a15a7fff650337906cb33de8042312c29 -r0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f
--- Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.resx	(.../Resources.resx)	(revision 9655570a15a7fff650337906cb33de8042312c29)
+++ Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Service/Properties/Resources.resx	(.../Resources.resx)	(revision 0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f)
@@ -150,4 +150,7 @@
   <data name="MacroStabilityInwardsInputValidator_ValidateZoneBoundaries_ZoneBoundaries_BoundaryLeft_should_be_smaller_than_or_equal_to_BoundaryRight" xml:space="preserve">
     <value>Zoneringsgrens links moet kleiner zijn dan of gelijk zijn aan zoneringsgrens rechts.</value>
   </data>
+  <data name="MacroStabilityInwardsInputValidator_ValidateTangentLines_TangentLineNumber_must_be_one_when_TangentLineTop_equals_TangentLineBottom" xml:space="preserve">
+    <value>Het aantal tangentlijnen moet 1 zijn wanneer tangentlijn Z-boven gelijk is aan tangentlijn Z-onder.</value>
+  </data>
 </root>
\ No newline at end of file
Index: Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Service.Test/MacroStabilityInwardsInputValidatorTest.cs
===================================================================
diff -u -r08a1d37783424699debf8c053dc95a75df38d8dc -r0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f
--- Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Service.Test/MacroStabilityInwardsInputValidatorTest.cs	(.../MacroStabilityInwardsInputValidatorTest.cs)	(revision 08a1d37783424699debf8c053dc95a75df38d8dc)
+++ Ringtoets/MacroStabilityInwards/test/Ringtoets.MacroStabilityInwards.Service.Test/MacroStabilityInwardsInputValidatorTest.cs	(.../MacroStabilityInwardsInputValidatorTest.cs)	(revision 0c5bcfd7a61efa90f83d11dd36c4d8582a78d63f)
@@ -360,6 +360,54 @@
             CollectionAssert.IsEmpty(messages);
         }
 
+        [Test]
+        public void Validate_MultipleTangentLinesWithTangentLineTopAndBottomSame_ReturnsError()
+        {
+            // Setup
+            input.TangentLineNumber = 3;
+            input.TangentLineZTop = (RoundedDouble) 1.5;
+            input.TangentLineZBottom = (RoundedDouble) 1.5;
+
+            // Call
+            IEnumerable<string> messages = MacroStabilityInwardsInputValidator.Validate(input).ToArray();
+
+            // Assert
+            CollectionAssert.AreEqual(new[]
+            {
+                "Het aantal tangentlijnen moet 1 zijn wanneer tangentlijn Z-boven gelijk is aan tangentlijn Z-onder."
+            }, messages);
+        }
+
+        [Test]
+        public void Validate_MultipleTangentLinesWithTangentLineTopAndBottomNotSame_ReturnsEmpty()
+        {
+            // Setup
+            input.TangentLineNumber = 4;
+            input.TangentLineZTop = (RoundedDouble) 1.5;
+            input.TangentLineZBottom = (RoundedDouble) 0.2;
+
+            // Call
+            IEnumerable<string> messages = MacroStabilityInwardsInputValidator.Validate(input).ToArray();
+
+            // Assert
+            CollectionAssert.IsEmpty(messages);
+        }
+
+        [Test]
+        public void Validate_SingleTangentLineWithTangentLineTopAndBottomSame_ReturnsEmpty()
+        {
+            // Setup
+            input.TangentLineNumber = 1;
+            input.TangentLineZTop = (RoundedDouble) 1.59;
+            input.TangentLineZBottom = (RoundedDouble) 1.59;
+
+            // Call
+            IEnumerable<string> messages = MacroStabilityInwardsInputValidator.Validate(input).ToArray();
+
+            // Assert
+            CollectionAssert.IsEmpty(messages);
+        }
+
         private static IEnumerable<TestCaseData> SurfacelineNotOnMacroStabilityInwardsSoilProfile2D()
         {
             yield return new TestCaseData(