Dropped letters are sorts of type that have fallen out or otherwise have been inadvertently removed from a page or pages of type in the locked form from which the sheet is printed. Sueltas tended to be indifferently printed and carelessly proof-read; as a result, dropped letters are not uncommon.  Sometimes the letters were pulled out of the form by the action of the leather balls used for inking the form; they are most likely to occur at the top or bottom of pages, esp. page numbers.

see also: broken type, font, "state" in edition, impression, issue, state