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 have to swear fealty to the throne. This is essentially the same as being a knight, but a Master of Arms can choose whether or not to swear fealty and usually is addressed as "Master" rather than "Sir". Much like a female knight is addressed as Sir, a female Master -at- Arms is addressed as Master rather than Mistress and is correctly referred to as such . 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 originally created because earlier SCA understandings of fealty held that Knights had to do whatever the Crown ordered, whether or not it was reasonable, sensible, or feasible. Over time it has also become a 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), or because of persona reasons for those purists from cultures with no real equivalent position.
While theoretically equal in rank to a Knight, in some places Mastery of Arms is frowned upon by the local Chivalry and/or populace. There was a major incident involving this dichotomy early in The Kingdom of Atenveldt during its Principality days.