How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified?

In Frequency Shift Keying (FSK), digital data is transmitted by shifting the carrier frequency between two distinct values. In radio and telecommunications terminology, these two frequencies are traditionally referred to as "mark" and "space." The "mark" typically corresponds to a binary 1 (the resting or active state), while the "space" corresponds to a binary 0. This terminology dates back to the days of mechanical teleprinters and telegraphy, where a "mark" literally meant making a mark on a paper tape, and a "space" meant leaving it blank. Distractors like "dot and dash" apply to Morse code (CW), while "on and off" applies to On-Off Keying (OOK), not FSK where a continuous signal simply shifts its frequency.