In *"Knots and Links"* by Rolfsen, he mentioned words like \*"the collar of a boundary", "bicollared boundary", "a bicollar on the boundary". I just wonder what the definition of "collar" is. Also, what a flat ball is?



Questions about a few terminologies in "Knots and Links" by Rolfsen
A *collar* is a small product neighbourhood of the boundary.


That is: Suppose that $M$ is a compact manifold. Let $\partial M$ be its boundary. A *collar* of $\partial M$ in $M$ is a submanifold $N \subset M$ homeomorphic to $\partial M \times [0, 1]$ so that $\partial M$ is the zeroth slice.


As for a flat ball, could you give an example of usage?


