Scott refers to the main catalogue used in the United States to describe and catalogue all the stamps of the world: the Scott Standard Postage Stamp Catalogue, 6 volumes, 1840-present. The US portion has been hived off and printed as a single volume with a LOT (huge) more detailed information in the Scott United States Specialized Catalogue. The worldwide material from 1840-1940 has been hived off into a single-volume Scott Classic Specialized Postage Stamp Catalogue. If you are doing post-1940 topicals, you almost have to use the 6 volume one. Used, out-of-date copies (2007, 2010 etc.) are availble on
ebay for anywhere from $15-50 a volume but even that adds up to a lot of money for a beginnner. It used to be that public libraries kept the current year's six volumes on hand--the cheap out-of-date copies on
ebay are often de-accessioned library copies that the library sold for a pittance when the new year's edition came in. Compact disks containing all 6 volumes of the Scott Standard Catalogue and selling for $9.95 on
ebay are pirated copies that violate the copyright held by Amos Advantage Publishing in Ohio, the current owner/publisher of the Scott Catalogues. The Scott empire was begun by a New York stamp dealer, John Walter Scott in the 1860s (first "catalogue" in 1868).
Scott numbers are the numbering system used in any of the above Scott catalogues. The first issue of the United States in 1847 is no. 1. The Panama-Pacific commemoratives of
1915 are nos. 398-401. The EInstein issue of 1979 is no. 1774. The Patsy Cline issue of 1993 is no. 2771. The Edgar Allan Poe issue of 2009 is 4377.
Characteristic of the Scott number system is that airmails, semipostals, postage due, parcel post etc. are placed after the "regular issues"-at the Back of the Book, hence "BoB" which you will see on
ebay and everywhere else. The regular issues get numbered from 1-4800 something (I only have a 2013 edition so I don't know what number the 2015 issues are at), then the Airmails all have a C: C1, C2, C57 and so forth. Then the Postage Dues all have a J number: J1, J7, J127 and so forth. Semipostals get a B number (B1, B7, B28) but the US issued virtually no semipostals. Since Scott Catalogue does cover the world, of course, it will have B number for Swiss semipostals, for German semipostals etc. Each country's listings start with a no. 1, so you can't just write, "Scott 13 or Scott 331." You have to specify "German Scott 13 or US Scott 24." However, on SCF, since it's heavily US dominated, people get lazy and write "Scott 534A," assuming everyone knows it's US Scott 534A. When posters on the Canada stamps section post without specifying, they usually mean Canada Scott no. 341 or whatever. Context, context, context.
The other two most extensive worldwide catalogue systems are, first, Michel from Germany, has differing number system that goes by years: regular issues, semi-postals, postage due, officials all get a running number. The second is Stanley Gibbons, from the UK, which does a bit of both--official stamps are integrated in the catalogue with the regular issues but get a "D1, D14" etc. number, while postage dues are put at the "back of the book" like Scott does.
Canada has the Unitrade Catalogue, which is highly preferred by Canadian specialists.
The US is dominated by the Scott system. UK and British Empire collectors almost always use Stanley Gibbons, European collectors swear by Michel. And there are other catalogues (Yvert for France and French Colonies and so on and so forth). But Scott, Michel, and Stanley Gibbons are the most comprehensive and widely used.
The StampWorld.com "catalogue" uses it's own numbering system, different from all the above.
We all wish there were at least a good concordance of these numbering systems but it's next to impossible (or else someone would have created it) because what one system treats as a major number another may treat as a variation. To actually line each system up alongside each other is nearly impossible.
If you want to specify a certain stamp in a manner that speaks to all users of all the catalogue systems, you describe it by year, color, denomination etc.: Scott US no. 1 would be United States 1847 5c red brown. Since only two stamps were issued in 1847, all you really need is 1847 5c.
But in some other year there might have been two or four 3c issued, so you have to specify color. If two of the same denomination and color were issued, you'd have to find something to distinguish (probably the motif).
You can see why it's easier to say "US Scott 301" or "Great Britain SG 5. Notice that SG gives 6 numbers (7-12) to what Scott calls GB no. 3.
For Scott, being US oriented, when it looks at Great Britain, it see merely this 1 penny red stamp from 1841 (Scott GB no. 3) and that 1 penny black from 1840 (Scott GB no. 1).
But for SG, in England, where these Great Britain stamps have been scrutinized in detail for a century and a half, there are 3 variants of the 1d black and each gets its own number, SG 1, SG 2, SG 3. Then there are 3 variants of 2 penny blue. For Scott it's merely GB no. 2 but for SG it's GB no 4, GB no 5, GB no 6. You get the picture!
An outsider looks what at Scott does with the simple 1c blue Franklin of 1851 and gags--Scott nos 5-9 and 18-24 (12 Scott numbers) are all basically the same stamp design but printed from a few different plates at different times between 1851 and 1861. The difference between 5-9 and 18-24 can depend on which part of the same plate the stamp came from--one part of the plate printed the side ornaments more or less completely than another part of the plate. Stamps fron the one part of the plate get a different Scott number than those from the other part of the plate--12 different variants.
edited for clarity