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All three pages of this card were based on a pattern sometimes called “tartan”, used for testing computer printers back in the days when they were line printers. This starts with a line formed of some regular and possibly repeated arrangement of characters; in the test pattern, this would have been a copy of the character set in the order it appeared on the printer’s chain or train, or perhaps some regular transformation of this. Following lines are then formed by the alternation of two simple transformations: in the first, each consecutive pair of characters is exchanged, with any odd character at the end of the line being held fixed; in the other, the other end character is (or both end characters are if the line is of even length) held constant and the rest of the characters are exchanged - now with different partners. The effect of this is that characters migrate down the page diagonally, alternate characters one way or the other, bouncing off the edge of the pattern when they reach it.
An interesting aspect of this card came about because it was formed simply by folding a piece of computer print-out. This meant that either the front (as here) or the whole inside of the card had to be upside down. Since computers were – in those days – not capable of printing upside down, this more or less required some crafty system of creating a message from inverted characters. Morse code is, of course, made of such characters.
This was a message in Morse code – details to come when I trace a copy.
These were plain text – details also to come.
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Version 24: Revised 22 December 2008