Devices that needed to produce output on printed forms would use a physical reference to ensure the text went to the right place. One of the commands a tabulato

Devices that needed to produce output on printed forms would use a physical reference to ensure the text went to the right place. One of the commands a tabulator could give the typewriter it drove was to move the form to the next marked position—a literal tab, or maybe a functional equivalent like a hope in a punched tape. Thus, vertical tab.