diff --git a/Semantics.Music/Arrangement.cs b/Semantics.Music/Arrangement.cs
index f778c1ea..1a14c682 100644
--- a/Semantics.Music/Arrangement.cs
+++ b/Semantics.Music/Arrangement.cs
@@ -33,6 +33,9 @@ other is not null
 	public double TotalBars => Sections.Sum(section => section.Bars);
 
 	/// <summary>Gets the structural form of the arrangement.</summary>
+	/// <exception cref="ArgumentException">
+	/// Thrown when the arrangement has more than <see cref="Form.MaxDistinctSections"/> structurally distinct sections.
+	/// </exception>
 	public Form Form => Form.Of(this);
 
 	/// <summary>Creates an arrangement.</summary>
diff --git a/Semantics.Music/Form.cs b/Semantics.Music/Form.cs
index eb30ec0d..a5abdc94 100644
--- a/Semantics.Music/Form.cs
+++ b/Semantics.Music/Form.cs
@@ -17,6 +17,9 @@ public sealed record Form
 		[1, 4, 1, 1, 4, 4, 1, 1, 5, 4, 1, 5],
 	];
 
+	/// <summary>The number of structurally distinct sections a form can label, one per letter A-Z.</summary>
+	public const int MaxDistinctSections = 26;
+
 	/// <summary>Gets the section letter-pattern (e.g. "AABA").</summary>
 	public string Pattern { get; private init; } = "";
 
@@ -42,7 +45,14 @@ other is not null
 	/// <summary>Derives the form of an arrangement from its sections.</summary>
 	/// <param name="arrangement">The arrangement to analyze.</param>
 	/// <returns>The derived form.</returns>
+	/// <remarks>
+	/// Each structurally distinct section takes the next letter from A to Z, so an arrangement can have at most
+	/// <see cref="MaxDistinctSections"/> distinct sections. Beyond that there is no letter <see cref="Parse"/> accepts.
+	/// </remarks>
 	/// <exception cref="ArgumentNullException">Thrown when <paramref name="arrangement"/> is null.</exception>
+	/// <exception cref="ArgumentException">
+	/// Thrown when <paramref name="arrangement"/> has more than <see cref="MaxDistinctSections"/> structurally distinct sections.
+	/// </exception>
 	public static Form Of(Arrangement arrangement)
 	{
 		Ensure.NotNull(arrangement);
@@ -55,6 +65,13 @@ public static Form Of(Arrangement arrangement)
 			int index = representatives.FindIndex(representative => representative.IsSameStructure(section));
 			if (index < 0)
 			{
+				if (representatives.Count == MaxDistinctSections)
+				{
+					throw new ArgumentException(
+						$"A form can label at most {MaxDistinctSections} structurally distinct sections (A-Z).",
+						nameof(arrangement));
+				}
+
 				representatives.Add(section);
 				index = representatives.Count - 1;
 			}
diff --git a/Semantics.Test/Music/FormTests.cs b/Semantics.Test/Music/FormTests.cs
index a4221874..242f47ad 100644
--- a/Semantics.Test/Music/FormTests.cs
+++ b/Semantics.Test/Music/FormTests.cs
@@ -40,6 +40,35 @@ public void Of_RecognizesTwelveBarBlues()
 		Assert.AreEqual(NamedForm.TwelveBarBlues, arrangement.Form.Name);
 	}
 
+	[TestMethod]
+	public void Of_TwentySixDistinctSections_RoundTripsThroughParse()
+	{
+		Arrangement arrangement = Arrangement.Create(CMajor, DistinctSections(Form.MaxDistinctSections));
+		Form form = arrangement.Form;
+		Assert.AreEqual("ABCDEFGHIJKLMNOPQRSTUVWXYZ", form.Pattern);
+		Assert.AreEqual(form, Form.Parse(form.ToString()));
+	}
+
+	[TestMethod]
+	public void Of_MoreDistinctSectionsThanLetters_Throws()
+	{
+		Arrangement arrangement = Arrangement.Create(CMajor, DistinctSections(Form.MaxDistinctSections + 1));
+		_ = Assert.ThrowsExactly<ArgumentException>(() => Form.Of(arrangement));
+	}
+
+	[TestMethod]
+	public void Of_RepeatsBeyondTwentySixSections_StillLabelsByStructure()
+	{
+		List<Section> sections = DistinctSections(Form.MaxDistinctSections);
+		sections.Add(sections[0]);
+		Form form = Arrangement.Create(CMajor, sections).Form;
+		Assert.AreEqual("ABCDEFGHIJKLMNOPQRSTUVWXYZA", form.Pattern);
+	}
+
+	// Sections with different chord counts never share a structure, so section i has i + 1 bars of C.
+	private static List<Section> DistinctSections(int count) =>
+		[.. Enumerable.Range(1, count).Select(bars => Section(SectionType.Verse, string.Join(" | ", Enumerable.Repeat("C", bars))))];
+
 	[TestMethod]
 	public void Parse_AppliesLetterRecognition()
 	{