The prehistory :
It was around 1960/67, as underpants we had models like this:
http://3.bp.blogspot.com/_5Av70BfofUY/S ... ourou_.jpg
Short on the legs and going up to the navel, quite loose, no support ... The shorts were only a daytime garment or a slack underwear "caleçons" worn by the grandpa

We only showered on Saturdays, so we wore the same underwear all week, night and day ... Yuck!

As a swimsuit, it was the grandma who knitted them for us ... in wool! It must have scratched us!
Fortunately later my mother bought us some store-bought swimsuits jerseys, the shape was rectangular (or square cuts?), like the current sunga from Brazil . In summer we wore only that, in the sun of the South of France and we became black skinned like Amazonians. And the grandma said that the color of our skin was rubbing off on the bed sheets ... It was filth in fact!

I don't really know when I wanted to wear smaller, triangular briefs or swimsuits, I felt the desire to feel thin, light or smooth fabrics like nylon against my skin. ... Like those of panties for big girls, if you will ... It was early adolescence, I think. Why did I have this desire and not my brothers or my friends? Who knows...
