We will now distinguish between two types of authentic ($\left.\text{V}\right.$–$\left.\text{I}\right.$) cadences: the Perfect Authentic Cadence (PAC) and the Imperfect Authentic Cadence (IAC).
1. $\left.\text{V}\right.$–$\left.\text{I}\right.$
3. Tonic scale degree ($\hat{1}$ ) in the highest voice of the tonic chord
If at least one of these requirements is not met (one or both of the chords are inverted, tonic is not in the highest voice, or $\left.\text{vii}^{\circ}{}\right.$ occurs in place of $\left.\text{V}\right.$), the cadence is an Imperfect Authentic Cadence (IAC).