2-sided

$hcolon\; Ftimes\; [-1,1]to\; M$

$h(Ftimes\; [-1,1])cap\; partial\; M=h(partial\; Ftimes\; [-1,1])$.

This means, for example that a curve in a surface is 2-sided if it has a tubular neighborhood which is a cartesian product of the curve times an interval.

A curve which is not 2-sided is called 1-sided.