Stradella basses have chord buttons for Major, Minor, 7th and Diminished chords. By using these chords in combination with different bass notes you can create other chord types, the most useful being Minor 7th, Major 7th, Half Dim (or m7b5), 9th chords (including b7th) and 6th chords (which I forgot to mention in the video but are on the chart).

Here is a chart showing all the most useful chord combinations relative to a given bass note which I call Tonic or 1. If the image is hard to read you can download and view or print the PDF at the bottom.

Accordion Stradella Bass Chord Combinations

You can download this image as a PDF: