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The Standard Catalog of World Paper Money is a well-known catalogue of banknotes that is published by Krause Publications in three volumes. These catalogues are commonly known in the numismatic trade as the Pick catalogues, as the numbering system was originally compiled by Albert Pick, but are also referred to as "Krause" or "SCWPM." 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.
The numbering system uses an integer to identify a note. The sorting of notes is usually by issue series/date, then ascending by denomination. Some varieties also have an alphabetic prefix, with a capital letter or letters.
In common usage, but not in the catalog itself, it is a common practice to prefix "P" to the catalog number (and any prefixes or suffixes, to designate that this is a "Pick" number; however, this is not a practice of the catalog themselves. If there are prefixes, in this usage, they will follow the "P" (e.g. "P5," "PS101a," "PM3" or "PFX").
Inconsistently, if a note has signature or date or other variants, then a lower case letter follows (e.g. P120a, P120b, P120c, etc.); in some cases though, multiple dates are assigned to a single variant; e.g. "1936-1940; 1942; 1945." From edition to edition, these variety letters may change, as additional dates and signatures are found, and as the editors decide to add more granularity.
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".
If a number needs to be inserted between two numbers, then in some cases, the section is renumbered--which creates confusion, and some collectors will annotate this with the previous number in parentheses .. e.g. 6(5) would indicate the current Pick number is 6, but it was once 5. But in other cases, the entries are not renumbered, and then the format is to use a suffix capital letter (e.g. P120A).
Most recent editions, as of August 2019.