Key spacing
Key spacing is the distance from the center of one key to the center of the next. MX-style boards use 19.05 mm, three quarters of an inch, called one unit or 1u. Boards made only for Kailh Choc switches usually space their keys 18 mm apart side to side and 17 mm front to back.
Keycap sizes use the same unit: a letter key is 1u, a 2u key is as wide as two. Cherry's datasheet puts the MX plate hole at 14 mm square, which leaves 5.05 mm of plate between switches.
Choc caps are not square. MBK caps are 17.5 mm wide and 16.5 mm deep, so 18 × 17 mm leaves half a millimeter between neighboring caps and makes a smaller board than MX spacing would.
Keycaps
Caps are made for one grid. MX-spaced caps won't fit an 18 × 17 grid, and Choc switches set 19.05 mm apart need Choc caps made for MX spacing. The "What fits this board" link on a design page sorts it out.
Choc v2 is the catch. The usual Choc-spaced caps, like MBK and Chosfox, have Choc v1 stems, so they do not fit Choc v2 and its cross stem. beekeeb's guide, from a shop that sells them, names Tai-Hao's MT 165 and THCS (16.5 × 16.5 and 17.5 × 16.5 mm in its listings). THCS also comes with Choc v1 stems, so check before you buy.
In this catalog
Of 338 designs, 257 use MX spacing, 50 use Choc spacing and 9 use something else. A few are below, each with the smallest distance between two switch centers on its PCB. Not recorded yet for 22 designs.
