Shift Out and Shift In characters

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 KOI-7, where SO starts printing Russian letters, and SI starts printing Latin letters again.

On many printers, however, SO starts printing double-width characters, and SI enters "condensed" mode (narrow characters).