What does a 'c' with a line under it mean?

An elevated 'c' with a line under it usually appears in surnames of Irish or Scottish decent. Although not as common today, many of Irish decent with the last name beginning with Mc (as in McNamara, McCarthy, McDonald, etc.) will write out their surname, elevating the 'c' and drawing a line under it. The reason for this is to show that a letter is missing from the name. For example, McNamara was previously spelled MacNamara, and McCarthy was previously spelled MacCarthy, and McDonald was previously spelled MacDonald. Leaving out the 'a', writers would substitute by elevating the 'c' and underlining it, drawing attention to the fact that the 'a' has been left out.

Years ago, you used to find surnames beginning with Mc right before the M section in a phonebook. For example, McGuil actually came before Martin. This is because the 'Mc' in McGuil is understood as 'Mac'.