Mottoes

Hello,
Does anyone have info on the mottoes that the nobility used? Were
they "required", more of a personal choice, or simply "decorative."
I seem to recall reading that Harry Buckingham's motto was "Often I
Remember", which to my mind sounds rather ominous, given his history!
What did the Duke of Clarence use, or any of the other lords from
our period? I am new to the forum, and have learned tons by lurking,
but it is time to come out from cover. Thanks in advance for any
response.