diff --git a/Semantics.Strings/Validation/Attributes/Format/HasExactLinesAttribute.cs b/Semantics.Strings/Validation/Attributes/Format/HasExactLinesAttribute.cs
index 3c6d8a2a..5a50cdf5 100644
--- a/Semantics.Strings/Validation/Attributes/Format/HasExactLinesAttribute.cs
+++ b/Semantics.Strings/Validation/Attributes/Format/HasExactLinesAttribute.cs
@@ -3,13 +3,13 @@
 namespace ktsu.Semantics.Strings;
 
 using System;
-using System.Linq;
 
 /// <summary>
 /// Validates that a string has exactly the specified number of lines
 /// </summary>
 /// <remarks>
-/// Line count is determined by counting line break characters plus one for the final line.
+/// Line count is the number of line breaks plus one for the final line. <c>\r\n</c> is one break, and so
+/// is a lone <c>\r</c>, <c>\n</c>, U+2028 or U+2029, matching <see cref="IsMultiLineAttribute"/>.
 /// Empty strings are considered to have 0 lines.
 /// A string with no line breaks has 1 line.
 /// </remarks>
@@ -56,16 +56,7 @@ protected override ValidationResult ValidateValue(string value)
 					: ValidationResult.Failure($"The text must have exactly {exactLines} line(s).");
 			}
 
-			// Count line breaks and add 1
-			int lineCount = value.Count(c => c == '\n') + 1;
-
-			// Handle Windows-style line endings (\r\n) - don't double count
-			if (value.Contains("\r\n"))
-			{
-				int crlfCount = value.Split(["\r\n"], StringSplitOptions.None).Length - 1;
-				int lfOnlyCount = value.Count(c => c == '\n') - crlfCount;
-				lineCount = crlfCount + lfOnlyCount + 1;
-			}
+			int lineCount = LineBreaks.CountLines(value);
 
 			bool hasExactLines = lineCount == exactLines;
 			return hasExactLines
diff --git a/Semantics.Strings/Validation/Attributes/Format/HasMaximumLinesAttribute.cs b/Semantics.Strings/Validation/Attributes/Format/HasMaximumLinesAttribute.cs
index a5475090..1dd52aaa 100644
--- a/Semantics.Strings/Validation/Attributes/Format/HasMaximumLinesAttribute.cs
+++ b/Semantics.Strings/Validation/Attributes/Format/HasMaximumLinesAttribute.cs
@@ -3,13 +3,13 @@
 namespace ktsu.Semantics.Strings;
 
 using System;
-using System.Linq;
 
 /// <summary>
 /// Validates that a string has at most the specified maximum number of lines
 /// </summary>
 /// <remarks>
-/// Line count is determined by counting line break characters plus one for the final line.
+/// Line count is the number of line breaks plus one for the final line. <c>\r\n</c> is one break, and so
+/// is a lone <c>\r</c>, <c>\n</c>, U+2028 or U+2029, matching <see cref="IsMultiLineAttribute"/>.
 /// Empty strings are considered to have 0 lines.
 /// A string with no line breaks has 1 line.
 /// </remarks>
@@ -53,16 +53,7 @@ protected override ValidationResult ValidateValue(string value)
 				return ValidationResult.Success(); // Empty strings have 0 lines, which is <= any positive maximum
 			}
 
-			// Count line breaks and add 1
-			int lineCount = value.Count(c => c == '\n') + 1;
-
-			// Handle Windows-style line endings (\r\n) - don't double count
-			if (value.Contains("\r\n"))
-			{
-				int crlfCount = value.Split(["\r\n"], StringSplitOptions.None).Length - 1;
-				int lfOnlyCount = value.Count(c => c == '\n') - crlfCount;
-				lineCount = crlfCount + lfOnlyCount + 1;
-			}
+			int lineCount = LineBreaks.CountLines(value);
 
 			bool hasValidLineCount = lineCount <= maximumLines;
 			return hasValidLineCount
diff --git a/Semantics.Strings/Validation/Attributes/Format/HasMinimumLinesAttribute.cs b/Semantics.Strings/Validation/Attributes/Format/HasMinimumLinesAttribute.cs
index 8aca2d81..8eca1d7c 100644
--- a/Semantics.Strings/Validation/Attributes/Format/HasMinimumLinesAttribute.cs
+++ b/Semantics.Strings/Validation/Attributes/Format/HasMinimumLinesAttribute.cs
@@ -3,13 +3,13 @@
 namespace ktsu.Semantics.Strings;
 
 using System;
-using System.Linq;
 
 /// <summary>
 /// Validates that a string has at least the specified minimum number of lines
 /// </summary>
 /// <remarks>
-/// Line count is determined by counting line break characters plus one for the final line.
+/// Line count is the number of line breaks plus one for the final line. <c>\r\n</c> is one break, and so
+/// is a lone <c>\r</c>, <c>\n</c>, U+2028 or U+2029, matching <see cref="IsMultiLineAttribute"/>.
 /// Empty strings are considered to have 0 lines.
 /// A string with no line breaks has 1 line.
 /// </remarks>
@@ -56,16 +56,7 @@ protected override ValidationResult ValidateValue(string value)
 					: ValidationResult.Failure($"The text must have at least {minimumLines} line(s).");
 			}
 
-			// Count line breaks and add 1
-			int lineCount = value.Count(c => c == '\n') + 1;
-
-			// Handle Windows-style line endings (\r\n) - don't double count
-			if (value.Contains("\r\n"))
-			{
-				int crlfCount = value.Split(["\r\n"], StringSplitOptions.None).Length - 1;
-				int lfOnlyCount = value.Count(c => c == '\n') - crlfCount;
-				lineCount = crlfCount + lfOnlyCount + 1;
-			}
+			int lineCount = LineBreaks.CountLines(value);
 
 			bool hasValidLineCount = lineCount >= minimumLines;
 			return hasValidLineCount
diff --git a/Semantics.Strings/Validation/Attributes/Format/IsMultiLineAttribute.cs b/Semantics.Strings/Validation/Attributes/Format/IsMultiLineAttribute.cs
index 090174b0..bd47d054 100644
--- a/Semantics.Strings/Validation/Attributes/Format/IsMultiLineAttribute.cs
+++ b/Semantics.Strings/Validation/Attributes/Format/IsMultiLineAttribute.cs
@@ -40,7 +40,7 @@ protected override ValidationResult ValidateValue(string value)
 			}
 
 			// Check for any line break characters
-			bool hasLineBreaks = value.Any(c => c == '\n' || c == '\r' || char.GetUnicodeCategory(c) == System.Globalization.UnicodeCategory.LineSeparator);
+			bool hasLineBreaks = value.Any(LineBreaks.IsLineBreak);
 			return hasLineBreaks
 				? ValidationResult.Success()
 				: ValidationResult.Failure("The value must contain line breaks.");
diff --git a/Semantics.Strings/Validation/Attributes/Format/IsSingleLineAttribute.cs b/Semantics.Strings/Validation/Attributes/Format/IsSingleLineAttribute.cs
index 01ed96eb..89fbf1df 100644
--- a/Semantics.Strings/Validation/Attributes/Format/IsSingleLineAttribute.cs
+++ b/Semantics.Strings/Validation/Attributes/Format/IsSingleLineAttribute.cs
@@ -40,7 +40,7 @@ protected override ValidationResult ValidateValue(string value)
 			}
 
 			// Check for any line break characters
-			bool isValid = !value.Any(c => c == '\n' || c == '\r' || char.GetUnicodeCategory(c) == System.Globalization.UnicodeCategory.LineSeparator);
+			bool isValid = !value.Any(LineBreaks.IsLineBreak);
 
 			return isValid
 				? ValidationResult.Success()
diff --git a/Semantics.Strings/Validation/LineBreaks.cs b/Semantics.Strings/Validation/LineBreaks.cs
new file mode 100644
index 00000000..cf504fb9
--- /dev/null
+++ b/Semantics.Strings/Validation/LineBreaks.cs
@@ -0,0 +1,45 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Semantics.Strings;
+
+using System.Globalization;
+
+/// <summary>
+/// The one definition of a line break that every line-related validation attribute shares, so that
+/// <see cref="IsSingleLineAttribute"/>, <see cref="IsMultiLineAttribute"/> and the <c>Has*Lines</c>
+/// attributes cannot disagree about how many lines a string has.
+/// </summary>
+internal static class LineBreaks
+{
+	/// <summary>
+	/// Reports whether a character ends a line: a line feed, a carriage return, the Unicode line
+	/// separator (U+2028) or the Unicode paragraph separator (U+2029).
+	/// </summary>
+	/// <param name="c">The character to check.</param>
+	/// <returns><see langword="true"/> if <paramref name="c"/> is a line break.</returns>
+	internal static bool IsLineBreak(char c) =>
+		c is '\n' or '\r'
+		|| char.GetUnicodeCategory(c) is UnicodeCategory.LineSeparator or UnicodeCategory.ParagraphSeparator;
+
+	/// <summary>
+	/// Counts the lines in a non-empty string: one more than the number of line breaks, where
+	/// <c>\r\n</c> is a single break and a lone <c>\r</c>, <c>\n</c>, U+2028 or U+2029 is one each.
+	/// </summary>
+	/// <param name="value">The string to count, which must not be empty.</param>
+	/// <returns>The number of lines in <paramref name="value"/>.</returns>
+	internal static int CountLines(string value)
+	{
+		int lines = 1;
+		for (int i = 0; i < value.Length; i++)
+		{
+			// The '\n' of a "\r\n" pair ends the same line its '\r' already counted.
+			bool secondHalfOfCrlf = value[i] == '\n' && i > 0 && value[i - 1] == '\r';
+			if (IsLineBreak(value[i]) && !secondHalfOfCrlf)
+			{
+				lines++;
+			}
+		}
+
+		return lines;
+	}
+}
diff --git a/Semantics.Test/LineCountValidatorsTests.cs b/Semantics.Test/LineCountValidatorsTests.cs
index a73a6c87..47f21b72 100644
--- a/Semantics.Test/LineCountValidatorsTests.cs
+++ b/Semantics.Test/LineCountValidatorsTests.cs
@@ -24,6 +24,22 @@ private sealed partial record Min2 : SemanticString<Min2> { }
 	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1812", Justification = "Used via generic type references")]
 	private sealed partial record Max2 : SemanticString<Max2> { }
 
+	[HasExactLines(3)]
+	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1812", Justification = "Used via generic type references")]
+	private sealed partial record Exact3 : SemanticString<Exact3> { }
+
+	[HasMaximumLines(1)]
+	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1812", Justification = "Used via generic type references")]
+	private sealed partial record Max1 : SemanticString<Max1> { }
+
+	[IsMultiLine]
+	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1812", Justification = "Used via generic type references")]
+	private sealed partial record MultiLine : SemanticString<MultiLine> { }
+
+	[IsSingleLine]
+	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1812", Justification = "Used via generic type references")]
+	private sealed partial record SingleLine : SemanticString<SingleLine> { }
+
 	[TestMethod]
 	public void HasExactLines_ZeroAndTwo_WithLfAndCrlf()
 	{
@@ -61,4 +77,39 @@ public void HasMaximumLines_Two_WithLfAndCrlf()
 		Max2 twoCrlf = SemanticString<Max2>.Create<Max2>("1\r\n2");
 		Assert.AreEqual("1\r\n2", twoCrlf.WeakString);
 	}
+
+	[TestMethod]
+	[DataRow("a\rb")]
+	[DataRow("a\nb")]
+	[DataRow("a\r\nb")]
+	[DataRow("a\u2028b")]
+	[DataRow("a\u2029b")]
+	public void EveryLineEndingCountsAsOneBreak(string twoLines)
+	{
+		Assert.AreEqual(twoLines, SemanticString<Exact2>.Create<Exact2>(twoLines).WeakString);
+		Assert.AreEqual(twoLines, SemanticString<Min2>.Create<Min2>(twoLines).WeakString);
+		Assert.AreEqual(twoLines, SemanticString<Max2>.Create<Max2>(twoLines).WeakString);
+		Assert.ThrowsExactly<ArgumentException>(() => SemanticString<Max1>.Create<Max1>(twoLines));
+	}
+
+	[TestMethod]
+	public void ConsecutiveCrlfPairsAreSeparateBreaks()
+	{
+		Exact3 three = SemanticString<Exact3>.Create<Exact3>("a\r\n\r\nb");
+		Assert.AreEqual("a\r\n\r\nb", three.WeakString);
+		Assert.ThrowsExactly<ArgumentException>(() => SemanticString<Max2>.Create<Max2>("a\r\n\r\nb"));
+		Assert.ThrowsExactly<ArgumentException>(() => SemanticString<Exact3>.Create<Exact3>("a\n\rb\r\nc"));
+	}
+
+	[TestMethod]
+	[DataRow("a\rb")]
+	[DataRow("a\nb")]
+	[DataRow("a\r\nb")]
+	[DataRow("a\u2028b")]
+	[DataRow("a\u2029b")]
+	public void LineCountsAgreeWithIsMultiLineAndIsSingleLine(string twoLines)
+	{
+		Assert.AreEqual(twoLines, SemanticString<MultiLine>.Create<MultiLine>(twoLines).WeakString);
+		Assert.ThrowsExactly<ArgumentException>(() => SemanticString<SingleLine>.Create<SingleLine>(twoLines));
+	}
 }