Tonic function

In harmony, a chord is said to have a tonic function if it feels “stable” and “at rest”, or that it doesn’t “feel the need to move to another chord”.^[1]

In diatonic music the chords that have a tonic function are $I$ and $V I m$ .^[1][2]

It’s worth noting that $I^{6}$ is not nearly as stable as $I$ and that $I_{4}^{6}$ is often actually of dominant function^[2] (also see: cadential 6-4).