Incomplete columnar transposition that alternates single letters and pairs.
The AMSCO Cipher is an incomplete columnar transposition. A keyword sets the number of
columns and the order in which they are read. The plaintext is written into the grid one row at a time,
but each cell takes an alternating chunk of one then two letters (or two then one). The ciphertext is read
off column by column in keyword order. See more information on
Wikipedia.