In, for example, Nairn where the population is about 4,000, I have over 300 marriages where a person of surname MAIN marries someone also named MAIN. And in a third of them, all the parents are also MAIN.
So to distinguish "which" MAIN the people are, there is a qualifier added which is a "tee-name".In the attached example the parents of Alexander and Christina MAIN who are the couple marrying are:
John MAIN (Duggie) and Anne MAIN (Ian)
and
William MAIN (Baillie) and Elizabeth MAIN (Bunker)
with tee names Duggie, Ian, Baillie, and Bunker showing that they come from four different strands of the MAIN family. I record them (e.g.) as John MAIN Duggie with the tee name Duggie held as a suffix to the main name which is MAIN.
