However, proportional fonts existed together with wide sentence spacing for centuries before the typewriter, and remained for decades after its invention. When the typewriter was first introduced, typists were most commonly taught to use three spaces between sentences. This gradually shifted to two spaces, while the print industry remained unchanged in its wide em-spaced sentences. Some sources now state it is acceptable for monospaced fonts to be single spaced today, although other references continue to specify double spacing for monospaced fonts. The double space typewriter convention has been taught in schools in typing classes, and that remains the practice in many cases. Some voice concern that students will later be forced to relearn how to type.