Master at Arms
In the SCA a Master at Arms (alternately a Master of Arms) is a peer of the Order of Chivalry who does not swear fealty to the throne. This is essentially the same as being a knight, but a Master of Arms usually is addressed as "Master" or "Mistress" rather than "Sir". The distribution between Knights and Masters-at-Arms is subject to quite a bit of regional variation, although in most Kingdoms, Knights greatly outnumber Masters.
The rank of Master at Arms was created in response to the very real concern that certain deserving individuals might restricted from the Chivalry because they could not swear the oaths, either from personal conviction or because of religious beliefs (i.e. the Jewish restriction on oath-swearing in the Kol Nidre)