It’s an archaic term from the days of typewriters, where a single push of the return lever would result in advancing the paper a single line space, appropriate for the font it was equipped with (this would frequently, but not always, be equivalent to 10-12pt monospaced type on 12pt lines). A second push would result in double spacing, and a control lever could be set to automate this or give one and a half line spaces.

Most word processing software has a simple toolbar function to simulate this. It gets a little more (needlessly) complicated the more sophisticated the software you use and the more knowledgeable you are.

Short answer:

Use a word processing program, set the font to Times New Roman, 14pt, and click the double-spacing icon on the toolbar. Let the software do the overthinking for you.

Is this a legal brief, by chance? I ask because you might as well be quoting from the Federal Rules of Appellate Procedure as to the requirements for briefs in the federal Courts of Appeals. In any case, if you’re using Microsoft Word, do NOT set “line spacing” to “double.” Use “Exactly 28.” In Word, “double” line spacing simply doubles the “single” line spacing, which builds in a small amount of leading. Court rules require “double spacing” as a holdover from the typewriter days—in these days of word processors, etc., it’s an ambiguous term. But “exactly 28” puts the lines too far apart as it is for good readability and aesthetics—setting “line spacing” at “double” just makes things worse.

If this is indeed a legal brief, or some other situation involving text on 6.5 inch lines and 8.5 X ll inch paper, I also recommend that you choose a font other than Times New Roman. It’s too condensed, even at 14 point. The best font on your computer, if it uses Windows XP or Vista, is probably Palatino Linotype. Dunno about Macs. Many local rules also let you use 13 point instead of 14. You should consider that if it’s an option.