The Standard Catalog of World Paper Money was a well-known catalogue of banknotes that was published by Krause Publications in three volumes.
Since the mid-1980s the titles have been owned by Krause Publications, and from 1994–2016 were under the editorship of George S. Cuhaj, and subsequently by Tracy L. Schmidt.
Inconsistently, if a note has signature or date or other variants, then a lower case letter follows (e.g. P120a, P120b, P120c, etc.
Where there are multiple versions of proofs, remainders or specimens, and occasionally other variations, they are often cataloged with a number after the letter, e.g. "p1", "p2", "s1" or "s2".
But in other cases, the entries are not renumbered, and then the format is to use a suffix capital letter (e.g. P120A).