Check letters

Check letters are on the stamps of Great Britain on most issues from 1840 to 1884. It identifies the exact position of stamp within a sheet starting with "AA" in the upper left corner. For example, the Penny Black sheet of 240 stamps is arranged "AA", "AB", "AC", etc. going across, ending in "AL". Going down the left margin, stamps are arranged "BA", "CA", "DA", etc., ending in "TA". Got it?

They can be used to detect forgeries when impossible letter combinations show up, as in the case of the Stock Exchange forgery. Check letters help identify plates in the Penny Black and Penny Red issues by their shape and position within their squares. See: plating.  File:GB 2d blue.jpg|position "RE". How did you guess? File:GB 3d plate 9.jpg|This is position "OK" and not "KO", okay? 

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