Refuse to label scale degrees the roman numerals cannot spell

Semantics.Music/Key.cs

@@ -101,11 +101,29 @@ public ScaleDegree FunctionOf(PitchClass root)
 	/// cased upper for major/augmented and lower for minor/diminished, then a quality suffix.
 	/// </returns>
 	/// <exception cref="ArgumentNullException">Thrown when <paramref name="chord"/> is null.</exception>
+	/// <exception cref="ArgumentException">
+	/// Thrown when the chord's root resolves to a scale degree beyond the seventh. Roman-numeral
+	/// analysis only spells seven degrees, so the eighth degree onwards — reachable in
+	/// <see cref="Mode.OctatonicHalfWhole"/>, <see cref="Mode.OctatonicWholeHalf"/> and
+	/// <see cref="Mode.Chromatic"/> — has no numeral to be given.
+	/// </exception>
 	public string RomanNumeralOf(Chord chord)
 	{
 		Ensure.NotNull(chord);
 
 		ScaleDegree degree = Scale.DegreeOf(chord.Root);
+
+		// Wrapping the degree onto the seven-entry table would hand two distinct degrees the same
+		// numeral, silently. Refuse instead, which is also what ChordFromRomanNumeral already does
+		// in the other direction rather than wrapping.
+		if (degree.Degree > RomanNumerals.Length)
+		{
+			throw new ArgumentException(
+				$"Degree {degree.Degree} of {Mode.Name} has no roman numeral: roman-numeral analysis " +
+				$"spells only {RomanNumerals.Length} degrees.",
+				nameof(chord));
+		}
+
 		StringBuilder sb = new();
 
 		if (degree.Alteration < 0)
@@ -117,7 +135,7 @@ public string RomanNumeralOf(Chord chord)
 			_ = sb.Append('#', degree.Alteration);
 		}
 
-		string numeral = RomanNumerals[(degree.Degree - 1) % RomanNumerals.Length];
+		string numeral = RomanNumerals[degree.Degree - 1];
 		bool lowerCase = chord.Quality is ChordQuality.Minor or ChordQuality.Diminished;
 		_ = sb.Append(lowerCase ? numeral.ToLowerInvariant() : numeral);
 

Semantics.Music/Progression.Analysis.cs

@@ -11,6 +11,10 @@ public sealed partial record Progression
 	/// <param name="key">The key to analyze against.</param>
 	/// <returns>One roman numeral per chord, in order.</returns>
 	/// <exception cref="ArgumentNullException">Thrown when <paramref name="key"/> is null.</exception>
+	/// <exception cref="ArgumentException">
+	/// Thrown when a chord's root resolves to a scale degree beyond the seventh, which has no
+	/// roman numeral. See <see cref="Key.RomanNumeralOf(Chord)"/>.
+	/// </exception>
 	public IReadOnlyList<string> RomanNumerals(Key key)
 	{
 		Ensure.NotNull(key);

Semantics.Test/Music/KeyTests.cs

@@ -36,6 +36,44 @@ public void RomanNumeral_ChromaticGetsAccidental()
 		Assert.AreEqual("bII", CMajor.RomanNumeralOf(Chord.Parse("Db")));
 	}
 
+	[TestMethod]
+	public void RomanNumeral_DistinctDegreesNeverShareALabel()
+	{
+		// The roman numeral table only spells seven degrees, so a mode with more of them has no
+		// label left for the eighth onwards. Wrapping would hand two distinct degrees the same
+		// string; the guard refuses instead, matching ChordFromRomanNumeral's own bounds check.
+		foreach (Mode mode in new[] { Mode.Chromatic, Mode.OctatonicHalfWhole, Mode.OctatonicWholeHalf })
+		{
+			Key key = Key.Create(PitchClass.Create(0), mode);
+			HashSet<string> seen = [];
+
+			for (int degree = 1; degree <= mode.DegreeCount; degree++)
+			{
+				Chord chord = Chord.Parse(key.Scale.PitchClasses[degree - 1].Name);
+
+				if (degree > 7)
+				{
+					_ = Assert.ThrowsExactly<ArgumentException>(
+						() => key.RomanNumeralOf(chord),
+						$"{mode.Name} degree {degree} has no roman numeral, so it must not be labelled.");
+					continue;
+				}
+
+				string numeral = key.RomanNumeralOf(chord);
+				Assert.IsTrue(seen.Add(numeral), $"{mode.Name} degree {degree} reuses the label '{numeral}'.");
+			}
+		}
+	}
+
+	[TestMethod]
+	public void RomanNumeral_ChromaticKeyStillLabelsTheFirstSevenDegrees()
+	{
+		Key chromatic = Key.Create(PitchClass.Create(0), Mode.Chromatic);
+		Assert.AreEqual("I", chromatic.RomanNumeralOf(Chord.Parse("C")));
+		Assert.AreEqual("II", chromatic.RomanNumeralOf(Chord.Parse("C#")));
+		Assert.AreEqual("VII", chromatic.RomanNumeralOf(Chord.Parse("F#")));
+	}
+
 	[TestMethod]
 	public void FunctionOf_ReturnsScaleDegree()
 	{