The imperforate stamps of Great Britain were printed in sheets of 240 (20 rows of 12) so that each row cost 1/- and an entire sheet cost £1.
In an attempt to prevent the illegal re-use of stamps each position on the sheet had a different check letters in the lower corners of the stamp.
The lower left corner denotes the row of the sheet that a stamp originated from, and is lettered from A to T
The lower right corner denotes the column of the sheet that a stamp originated from, and is lettered from A to L
So a sheet consists of stamp “AA” in the top left corner, and stamp “TL” in the bottom right corner.
Using these letters it is possible to re-construct what a sheet would have originally looked like.