diff --git a/Semantics.Music/Chord.cs b/Semantics.Music/Chord.cs
index 1f37c03..fb74390 100644
--- a/Semantics.Music/Chord.cs
+++ b/Semantics.Music/Chord.cs
@@ -312,14 +312,19 @@ private static SeventhType DetermineSeventh(string body, ChordQuality quality)
 		bool hasSeven = body.Contains('7');
 		bool hasMaj7 = body.Contains("maj", StringComparison.Ordinal) || body.Contains("M7", StringComparison.Ordinal) || body.Contains('Δ');
 
-		if (quality == ChordQuality.Diminished && body.Contains("dim", StringComparison.Ordinal) && hasSeven)
+		// The major seventh is tested first because one body can carry both spellings: "dimmaj7"
+		// is the diminished-major seventh, a chord distinct from "dim7", and it is what the
+		// formatter emits for Diminished + Major. Testing the diminished branch first swallowed
+		// it, since "dimmaj7" contains "dim" and a '7' exactly as "dim7" does. "dim7" itself is
+		// unaffected: it carries no "maj".
+		if (hasMaj7)
 		{
-			return SeventhType.Diminished;
+			return SeventhType.Major;
 		}
 
-		if (hasMaj7)
+		if (quality == ChordQuality.Diminished && body.Contains("dim", StringComparison.Ordinal) && hasSeven)
 		{
-			return SeventhType.Major;
+			return SeventhType.Diminished;
 		}
 
 		return hasSeven ? SeventhType.Dominant : SeventhType.None;
diff --git a/Semantics.Test/Music/ChordRoundTripTests.cs b/Semantics.Test/Music/ChordRoundTripTests.cs
index fb990dd..6b81264 100644
--- a/Semantics.Test/Music/ChordRoundTripTests.cs
+++ b/Semantics.Test/Music/ChordRoundTripTests.cs
@@ -28,6 +28,24 @@ public void CanonicalOutputRoundTrips()
 		}
 	}
 
+	[TestMethod]
+	public void DiminishedMajorSeventhRoundTrips()
+	{
+		// The corpus above cannot cover this one. It starts from a symbol, and "Cdimmaj7" parsed
+		// wrongly as a diminished seventh still formats and re-parses consistently as "Cdim7" —
+		// a stable round trip of the wrong chord. The chord object is the only honest starting
+		// point, since it is what the formatter is being asked to be the inverse of.
+		Chord chord = new() { Quality = ChordQuality.Diminished, Seventh = SeventhType.Major };
+
+		Assert.AreEqual("Cdimmaj7", chord.ToString());
+
+		Chord reparsed = Chord.Parse(chord.ToString());
+
+		Assert.AreEqual(SeventhType.Major, reparsed.Seventh);
+		Assert.AreEqual(ChordQuality.Diminished, reparsed.Quality);
+		Assert.AreEqual(chord, reparsed);
+	}
+
 	[TestMethod]
 	public void TryParseReturnsFalseOnEmpty()
 	{
diff --git a/Semantics.Test/Music/ChordTests.cs b/Semantics.Test/Music/ChordTests.cs
index edc323f..e83466f 100644
--- a/Semantics.Test/Music/ChordTests.cs
+++ b/Semantics.Test/Music/ChordTests.cs
@@ -42,6 +42,33 @@ public void Parse_MinorMajorSeventh_IsBothMinorAndMaj7()
 		Assert.AreEqual(SeventhType.Major, c.Seventh);
 	}
 
+	[TestMethod]
+	public void Parse_DiminishedMajorSeventh_IsDiminishedTriadWithMajorSeventh()
+	{
+		Chord c = Chord.Parse("Cdimmaj7");
+		Assert.AreEqual(ChordQuality.Diminished, c.Quality);
+		Assert.AreEqual(SeventhType.Major, c.Seventh);
+	}
+
+	[TestMethod]
+	public void Parse_DiminishedSeventh_StillHasTheDiminishedSeventh()
+	{
+		// The guard on the reordered checks: "dim7" carries no "maj", so it must not be swept
+		// into the major-seventh branch that now runs first.
+		Chord c = Chord.Parse("Cdim7");
+		Assert.AreEqual(ChordQuality.Diminished, c.Quality);
+		Assert.AreEqual(SeventhType.Diminished, c.Seventh);
+	}
+
+	[TestMethod]
+	public void ChordTones_DiminishedMajorSeventh_HasTheMajorSeventhNotTheDoubleFlatSeventh()
+	{
+		// The audible half of the bug: a misparse puts the bb7 (9) where the major 7th (11)
+		// belongs, so Voice() and playback are a semitone out on that tone.
+		int[] actual = [.. Chord.Parse("Cdimmaj7").ChordTones()];
+		Assert.AreSequenceEqual([0, 3, 6, 11], actual, "Cdimmaj7 should be a diminished triad with a major seventh.");
+	}
+
 	[TestMethod]
 	public void Parse_HalfDiminished_IsDiminishedTriadWithDominantSeventh()
 	{