תהילים לא:כד פירוש: רש״י

The Lord guards those who believe in His salvation and rely on Him.

with a bowstring Heb. יתר (with measure for measure and with exactitude, like an arrow on a bowstring. Not in all editions.) Or, יתר can be interpreted as a rope for a rope, a line for a line. Another interpretation: יתר is an expression of haughtiness, as (in Isa. 15:7): “Because of the haughtiness (יתרה) with which they acted.”