Calt is an altered dominant: it keeps the root, major 3rd, and flat 7th of , then changes upper notes. This page spells it C, E, Bb, D# and Ab.
Here the altered notes are and . “Alt” is a family name, so another usable voicing can choose different alterations.
Also written C7alt, C7altered or C altered dominant.
Calt in root position puts C in the bass, which is the chord as it is spelled: the note it is named after underneath everything else.
5 of Calt’s 9 shapes do this:x32344x32342x3232287889x8x8999
Alt is not one chord but an instruction: a dominant with its ninth and fifth altered. Its usual home is between Gm7b5 and Fm6, 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 Calt works as the V of the ii: Ebmaj7, Calt, Fm7, Bb7. 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.
Contains Calt and adds flat ninth and flat fifth.
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.