Printers' devices or trademarks
Almost from the outset printers began marking their books with distinctive trademarks, bearing their arms, initials, name or another distinctive mark to claim credit for the work they had done. The first device used was that of Fust and Schoeffer in a Psalter of 1457 where it appears as part of the colophon. The colophon was the brief description at the end of a book providing the names of the author and the printer or scribe and the date. It had been used by scribes in handwritten manuscripts and in common with other manuscript features it continued to be used by early printers.
The final page of the unique Lyme Missal (1487) bears Caxton's printer's device - the first time the printer marked one of his books in this way. As an English merchant, Caxton was exempt from some of the import duty paid by foreigners. He would therefore have marked his goods with a merchant's stamp to identify them. His printer's device is unusually large and may be his actual merchant's mark.
The device of a particular printer became synonymous with their reputation and craftsmanship and unscrupulous rivals would adopt similar devices to try and outwit the market.
Later presses hung signs displaying their device outside their workshops. This in turn led to the phrase 'Printed at the Sign of the _____' being printed on the title page of books.