The Apostrophe ['].
This punctuation mark does three things:
Two contradictory trends -- both bad -- are at work with apostrophes.
First, careless writers want to form plurals with wayward apostrophes -- e.g.: "The bishop's [read 'bishops'] of the United Methodist Church have issued an urgent appeal for funds to assist the victims of flooding in the Midwest." Monte Marshall, "Special Offering for Flood Relief," United Methodist Rep., 3 Sept. 1993.
The second unfortunate trend is to drop necessary apostrophes: there is a tendency to write "the hotels many shops" or "Martins Pub." The only possible cure is increased literacy.
Finally, U.S. place names drop possessive and associative apostrophes by government policy. So, what was once Harper's Ferry, West Virginia, became Harpers Ferry.