Figures, Letters, and Unshift On Space (USOS)
RTTY uses the 5-bit Baudot code. 5 bits can represent 25 or 32 characters, but 32 characters isn't enough to convey 26 letters, 10 digits, and assorted punctuation and other symbols. Monsieur Baudot solved this problem by defining two banks of characters. The Letters bank conveys the 26 letters, space, carriage return, and line feed characters. The Figures bank conveys the numbers, space, punctuation, and symbol characters. A Shift to Letters character means interpret all subsequent characters as letters, and a Shift to Figures character means interpret all subsequent characters as Figures. Thus the 5-bit character 00011 conveys the letter A if the last shift character received was Shift to Letters, or the symbol - (hyphen) if the last shift character received was Shift to Figures. These shift characters are often abbreviated as LTRS and FIGS.
The problem with this scheme is that failure to receive a LTRS or FIGS character due to QRM, QRN, or QSB causes all subsequent characters to be misinterpreted. The sequence
11011, 10000, 11000, 11000
which is FIGS 5 9 9 will be decoded as TOO if the FIGS character is not successfully received.
One way to minimize the damage caused by missing Shift characters is a policy called Unshift on Space, abbreviated USOS. When this policy is in force, the receiver interprets a received space character to mean receive a space character and shift to Letters. A transmitter must know that a receiver has (or may have) USOS enabled so that it appropriately inserts FIGS characters after spaces when sending text like
123 456 789
These FIGS characters would not be required in the absence of USOS and do reduce transmission speed, but improve resilience in the face of interference or fading if USOS is enabled. You can enable USOS on the RTTY tab of WinWarbler's Configuration window.
Despite USOS, garbled LTRS or FIGS characters can cause a word to be misdecoded -- TOO being a common example. To decode less common examples, clicking on a received word in WinWarbler's Receive pane while depressing the Ctrl key will replace the each character in that word with its analog in the opposite bank, and toggle the character's underlining in the Receive Pane. If you receive a garbled word, try Ctrl-clicking it; if that doesn't make it intelligible, Ctrl-click it again to return it to its original state.