This is the concept of fashion shape between the 70s and 80s...it bagan in the 70s with the wider clothes at the bottom of the body(flares etc.) and the narrow clothes at the top.
In the 80s the triangle reversed its self...i.e skinny jeans and shpulder padding coupled with larger hair.
Nowadays, the triangle can be worn in any direction and called 'trendy'.
Colin: Hey what do you think of my new jumper? with thease killer triangles im gonna be like sooooo trendy d00d!!
Amy: Hey you soo totally got the wrong idea about this whole fashion triangle thing wrong! That knit with triangle deatailing is so like...bad.
Colin: Oops im gonna burn it now and purchase some REAL trendy clothes!