The letters keep their identity but are shuffled by reading a grid column by column, in keyword order.
transposition cipher
easy to break by hand
First World War
Field ciphers; made famous by ADFGVX and the SOE
encrypt · decrypt · cryptanalysis
The ciphertext was cut into columns and stacked. Its length must divide evenly by the column count — pick a candidate; the match score says how English-like the best column arrangement looks.