Definition: A horizontal mintmark is one that has been placed on a  working die on its side.

From the beginning, when mintmarks were punched into the working die by hand, until the end in the year 1989 when that practice ceased, some mintmarks were accidentally punched into the working die in a horizontal position.

In all cases, the incorrect mintmark was punched over with a mintmark that in the proper vertical position. The images to the right show a horizontal S under a vertical S-mintmark on 1909-S Lincoln cent, a horizontal D under a vertical D-mintmark on a 1950-D Lincoln cent, and a horizontal D under a vertical D-mintmark on a 1961-D Lincoln cent.