The start frame delimiter, sent in reverse bit order from the long preamble's.
Reversed deliberately, and this is the whole detection mechanism: a receiver that finds the long delimiter knows it has a long preamble, and one that finds the reversed pattern knows it has a short one. Nothing announces which format is coming — the delimiter itself is the announcement.
SFD
Authored in HR/DSSS short preamble PPDU . Drawn in 20 frames.