There may be a bridge or brace that connects the stays above the rear wheel and below the connection with the seat tube.
The seat tube contains the seatpost of the bike, which connects to the saddle.
Hence designs that lack a seat tube or top tube are not allowed.
The Gekko folds from the seat tube like an upside down umbrella.
The most common location for a second frame-mounted bottle cage is on the front side of the seat tube.
Soon the seat tube was added, creating the modern bike's double-triangle diamond frame.
This helps to prevent water from running down the seat tube.
An alternative solution would be to use an E-type front derailleur, which does not clamp around the seat tube at all.
The vast majority of front derailleurs are mounted to the frame by a clamp around the frame's seat tube.
It features his portrait on the seat tube.