Add a C# on the 4th fret of the A string when playing an open A chord to create an A/C# chord. We’re simply changing the lowest note to C# to create an A/C# chord.

Use your 1st finger as a barre across the D, G, and B strings at the 2nd fret and use your 2nd finger to grab that C note on the A string. It’s an A chord, but we add the C as the lowest note to create a A/C chord.