The reason dance practice clothes and swimsuits for girls are triangular at the hip is because this does not cause discomfort when the legs move up and down. This design comes primarily from the shape that the human body molds itself to when it moves naturally. When lifting the leg forward, the junction between the thigh root and the pelvic bone has to give way in order for the leg to be lifted higher. These two garments are also built to cater to this naturally occurring line of the human body. I hope my answer can help you.