inked marking string; snap line
A string coated with ink, used in carpentry and construction to mark straight lines on wood or other materials. Often used with a sumitsubo (ink pot).
The carpenter used an inked marking string to draw a straight line on the lumber.
Compound of 墨 (sumi, 'ink') and 糸 (ito, 'thread'). The term directly describes the inked thread used in traditional carpentry.