Unch

The Sympathy Crossword Grid Editor supports: ... o flagging of lights outside a configurable unch range

Source: wiktionary

From 1933 onwards the grids were standard grids. Then, for some reason, there was a blip in the late 1940s, a couple of the grids from that era had double unches and double checking.

Source: wiktionary

The bars and multitude of black squares are common in British puzzles because they allow unchecked letters (called unches)—or letters that don't need to cross with another answer.... There are rules regarding the unches, and traditionally every other letter in a solution should be checked with another letter.

Source: wiktionary