120–145% of the point size
Line spacing is the vertical distance between lines of text. Most lawyers use either double-spaced lines or single-spaced lines—nothing in between.
These are obsolete typewriter habits. Originally, a typewriter’s platen could only move the paper vertically in units of a single line. Therefore, line-spacing choices were limited to one, two, or more lines at a time. Double-spacing became the default because single-spaced typewritten text is dense and hard to read. But double-spacing is still looser than optimal.
Most courts adopted their line-spacing standards in the typewriter era. That’s why court rules usually call for double-spaced lines. On a typewriter, each line is the height of the font, thus double spacing means twice the font size. So if you’re required to use a 12-point font, double line spacing means 24 points.