License plate formats can vary greatly from state to state. Above all, the goal is to use a format with enough alphanumeric character combinations to ensure that every vehicle in the state can have a unique number.
Less populous states may be able to accommodate motorists with simple numeric formats up to six digits. Most license plates, however, tend to use six- or seven-character combinations of letters and numbers. For smaller states, a seven-character format provides the flexibility to incorporate codes for things such as counties and expiration dates.
Even printing methods vary; most states currently emboss or press the registration number into the plate, but digital printing (or “flat plate” technology) is increasingly popular. About the only thing that North American (including Canadian) license plates have in common is size: In 1956, the US and Canada agreed that all license plates would measure roughly 6x12".