[throwawayish]

There are also other interesting uses for odd word lengths. For example, many UARTs support word lengths from 5 to 9-ish bits (some do more). This is commonly used to implement out of band signalling for protocols running over these, eg. using 9 bit words, where the ninth bit tells whether this is the start of a command frame. More handily even, in most MCUs this is already correctly separated, ie. there is a byte register with the lower 8 bits and then there is a different register for the remaining bits.

[Zardoz84]

Or using the 9th bit to indicate that the rest of the byte is an address . Some PICs UARTS had an interruption that is triggered when the 9th bit is on.