How the straddling checkerboard works

The most common letters sit in the top row of a grid and get a single digit each. Two columns in that row are left blank, and the remaining letters go in two lower rows labeled with those blank digits, so they get two digits. Since any 2 or 6 (in the usual layout) starts a two-digit letter, the message can be read without separators. Soviet spies used it, often followed by another step, and it is part of the famous VIC cipher.

Set the alphabet in order, top row first, and which two columns are blank. A / in the alphabet marks where numbers follow.