The prisoner's constraint is a lipogrammatic rule that prohibits the writer from using letters with ascenders or descenders.1 In other words, the writer isn't allowed to use b, d, f, g, h, j, k, l, p, q, t, or y.2 The remaining, allowed letters—a, c, e, i, m, n, o, r, s, u, v, w, x, z—all have the same height (except for "i"). A line of them ("some mean necromancers menace an orc sorceror") is smooth on the top and bottom.
Without ascenders or descenders, the writing produced under this constraint conserves vertical space3—something a prisoner would have in short supply.
6,098 of the 173,528 words on the ENABLE2K list are prisoner's constraint-legal. The two longest are "semiconsciousnesses" and "unceremoniousnesses", each 19 letters.
I get a pleasant sense of austerity from just looking at prisoner's constraint writing, especially if it's monospaced. Like the feel of a worry stone, or "goggles" typed on the seven-segment display of a calculator.
1"Ascender" and "descender" are typographical terms referring to portions of letters that extend above or below a font's mean line or baseline, respectively. The vertical stem of "t", for example, is an ascender, while the vertical stem of "p" is a descender.
2If you ask me, "i" shouldn't be allowed either, on account of the dot.
3Actual space savings will vary upon your font or handwriting.