Slash Chord Notation
You will notice – that I am using slash chord notation even when I write out the full accordion arrangements. The reason I am choosing to do this – is to get you accustom to seeing it. Then when you see it in piano/vocal music or jazz notation – it won’t freak you out.
The simplest explanation is – the top or 1st letter is the chord or triad and the bottom or 2nd number (after the slash) is the bass note. SO if you see C7/Bb – you are to play the C7th chord – but instead of the C bass – you will play the Bb bass.
For a very thorough explanation – please open this link.