Diamond frame and ladies safety bicycles

Diamond frame safeties can be divided into three types:
- in the beginning, there was no seat tube at all.
- after a few years, there came a proper seat tube, it was cuved to build a short frame.
- finally the seat tube became straight.
- then, there are the special frames: double tubes, semi tubular etc. 

No seat tube
 Curved seat tube
Straight seat tube
 Special frames, double tubes etc.