When speaking or writing in the third person, sir was used as a prefix to names of knights and priests. In direct address it was used much more broadly: in the Canterbury Tales the innkeeper is addressed as sire hoste, and a monk, friar, canon, summoner, cook, doctor, man of law, yeoman and Chaucer himself are all addressed as sir.
In the Paston letters master is used as a third person prefix for clerks, either lay or religious, as well as when the writer is an employee of the person described. Where the individual is identifiable these seem to have been the only uses in the letters. The records of Mercer's Company of London used master in the same way, and as a prefix for officers of the gild: the masters of the gild, aldermen and wardens. Ordinary masters of apprentices who were not officers were named without the prefix. Civic aldermen and mayors also received the prefix of master.