Scottish and Irish patronymic surnames frequently have the prefix Mac or Mc. When these surnames were originally developed, they were formed by adding the Gaelic word mac, which means son of, to the name of the original bearer's father. For example, the surname MacDougall literally means son of Dougal. In later times, these prefixes were also added to the occupation or nickname of the bearer's father. For example, MacWard means son of the bard and MacDowell means son of the black stranger.

Numerous variations of this prefix emerged, for a number of reasons. It was rendered Mag before vowels and aspirated consonants. Historical records concerning Irish and Scottish names reveal that the common prefix Mc and the less common prefixes M' and Mcc developed as abbreviations of the original Gaelic prefix Mac. Thus, the popular beliefs that Mc is a distinctively Irish prefix while Mac is exclusively Scottish, and that one prefix is used by Catholic families while the other one is specifically Protestant are erroneous. In actuality, the same person often had his surname recorded using both Mac and Mc on separate occasions.