What type of modulation is used by FT8?

FT8 is a highly popular digital mode designed by Joe Taylor and Steve Franke for making contacts in weak-signal or noisy conditions. The '8' in FT8 gives away exactly how the modulation works: it uses 8 discrete audio tones. The transmission rapidly shifts among these 8 distinct frequencies to encode digital data, which defines it as 8-tone frequency shift keying (FSK). Each tone shift happens continuously and phase-coherently, occupying a very narrow bandwidth of just 50 Hz. The distractors rely on buzzwords. Vestigial sideband is for television, while direct sequence spread spectrum is a wideband technology used in modern Wi-Fi and military communications, completely opposite to the narrowband nature of FT8.