Dalt is D, F#, C, E# and Bb — with every tension it can carry altered at once.
It sounds maximum tension, barely holding together. Not one chord but an instruction: take the root, third and flat seventh, add any two alterations, and let the rest go.
Altered dominant — the hardest pull available.
Also written D7alt, D7altered or D altered dominant.
Drop the third voice from the top an octave instead. The wider gap skips a string, so the muted string is part of the shape.
That result, 3–R–5–7, is the shape usually meant by drop 3. Named by its bass note, as the inversions are here, it is the 1st inversion.
More familiar jazz grips are drop 3 than drop 2.
Alt is not one chord but an instruction: a dominant with its ninth and fifth altered. Its usual home is between Am7b5 and Gm6, where the ♭13 is already the minor third of the chord it resolves to.
Listen for: The ii’s minor third falls a whole step to the V’s ♭13, which holds as the minor third of the i.
In a major key Dalt works as the V of the ii: Fmaj7, Dalt, Gm7, C7. Its ♭13 is the minor third of the ii it leads to.
Listen for: The top voice rises a half step to the ♭13 and then stays put: minor third of the ii, flat seventh of the V.
Every note of Dalt is in D altered.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Dalt sits in it. Every scale.
Change one note of Dalt and you have (drop the sharp ninth), (the sharp ninth becomes the ninth) or (the flat thirteenth becomes the fifth).
Dalt is an altered chord — a dominant seventh with its fifth or ninth moved. Altered dominant chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.