What is a I6 chord?

A I6 is the analytical way to say "I chord, first inversion." This could also mean (and it usually does) that the I chord, in any inversion, has the third in the bass. Let's look at some examples:

Suppose you were in the key of C major. The I chord would be C, made up of the notes C, E, and G with C in the bass. Therefore, the I6 chord would be C/E (C with the bass note E).

Suppose you were in the key of G major. The I chord would be G, made up of the notes G, B, and D with G in the bass. Therefore, the I6 chord would be G/B (G with the bass note B).

This also brings up if you have the I chord with the fifth in the bass. That is written as I46 (but with the numbers directly on top of each other, which I don't know how to do on the computer). In C this chord would be C/G.