Shift Out (SO) and Shift In (SI) are ASCII control characters 14 and 15, respectively (0xE and 0xF).  The original meaning of those characters was to switch to a different character set and back.  This was used, for instance, in the Russian character set known as KOI7, where SO starts printing Russian letters, and SI starts printing Latin letters again.

Some older printers used these characters to control special features, such as changing the font or ink color. On the Model 38 Teletype, SO switched to red printing while SI switched back to black printing.

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