Kentō are the registration marks used to align color blocks in ukiyo-e printing.
A printer carved small guide marks into the edge of the key block and each successive color block. The paper was placed against these marks so outlines and colored areas met accurately. The system was essential to multiblock printing, especially after full-color nishiki-e became established in the eighteenth century.
Kentō normally included an L-shaped corner mark and a straight-line mark. Their positions could be trimmed away or hidden when the finished print was mounted. They are not part of the pictured design itself.
A common mix-up is confusing kentō with the key block. The key block carries the principal outlines; kentō only controls alignment. Precise registration allowed printers to combine many separate impressions on one sheet without visible shifts.