Wikipedia

In the early 1970s, the developers of the yet-to-be-built mall gave the land for the tower to the city, stipulating that the words "Florence Mall" be painted on it in view of interstate motorists. The giant advertisement immediately raised legal concerns. In July 1974, state Bureau of Highways officials told the city that the tower's sign was illegal because it advertised something which did not yet exist.

With time running out to comply with the law, civil staff met for a brainstorming session at the Stringtown Restaurant with C.M. "Hop" Ewing, then mayor of Florence, who "sketched different ideas on a napkin". Ewing ultimately devised the idea of removing the vertical lines at the sides of the M in MALL, adding a stem to make it a Y and adding an apostrophe; resulting in "Y'all". Ewing called it a "corny solution, but cost-effective", because the minor alteration would cost one-third of a full repainting. The city paid $472 to the W.T. Marx Company of Cincinnati, Ohio, to make the changes.