the.com/gridline

The faint lines that let you lie precisely instead of guessing wildly.

means One of the faint reference lines on a graph or map that let you measure and align things accurately.

from A straightforward compound of 'grid' (from 'gridiron,' the medieval cooking rack of parallel bars, later borrowed for any network of crossing lines) and 'line.' The sense of 'grid' as a graph or map framework is a 20th-century extension — the cooking rack's crosshatch pattern lending its name to anything similarly ruled into squares.

origin
From cartography and graph paper, not design software
job
Marks scale and position without demanding attention
design rule
Good gridlines are seen only when searched for
excel fame
Most people only know them from spreadsheets
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