Bbalt is an altered dominant: it keeps the root, major 3rd, and flat 7th of , then changes upper notes. This page spells it Bb, D, Ab, C# and Gb.
Here the altered notes are and . “Alt” is a family name, so another usable voicing can choose different alterations.
Also written Bb7alt, Bb7altered or Bb altered dominant.
Alt is not one chord but an instruction: a dominant with its ninth and fifth altered. Its usual home is between Fm7b5 and Ebm6, 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 Bbalt works as the V of the ii: Dbmaj7, Bbalt, Ebm7, Ab7. 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 Bbalt 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.