Mac, Mc, and M'

1st Street? First Street?
M'Donald? McDonald? MacDonald? Macdonald?

19th Century librarians tried to impose order and consistency on a chaotic universe. They championed uniformity of practice so that library users could anticipate consistency from one institution to the next, and so that catalogs and indexes would be kept orderly through gradual changes in library staff.

Those practices became institutionalized and codified. One of those uniform practices was "regularization" of M', Mc, and Mac so they all were interfiled as if they were spelled "mac". Quick -- how's the "golden arches" restaurant spell its name? Following this rule it would not make any difference.

But like simplified spelling and other manifestations of 19th Century orderliness, these rules never found widespread adoption in the world at large. Now we live in the world of the ASCII sort, where the Lincoln telephone book has successive listings for Mazza, Mbiya Bondo, MC2, McAdams, and McAlister.
Following that trend, Kilgore Memorial Library is abandoning a hoary tradition -- Macarthur and McArthur are parting company; Macdonald and McDonald are splitsville.