What is a 'horizontal' versus 'vertical' format sometimes used on IDs to indicate?

In many states, driver's licenses and ID cards for people under 21 are printed in a vertical (portrait) orientation, while IDs for those 21 and older are printed horizontally (landscape). This visual cue helps servers and sellers quickly flag a potentially underage customer, prompting a careful check of the date of birth. However, the orientation is only a cue, not a substitute for verifying the actual date of birth, and formats vary by state, so servers should always confirm age from the birth date itself.