TITLE: What does this (double absolute value like) notation mean?
QUESTION [18 upvotes]: Here,
$$\left\lVert\frac{\partial\bf x}{\partial s}\times\frac{\partial\bf x}{\partial t}\right\rVert$$
the inside will at last be a vector. and two absolute value signs have covered it. what does it mean?
Can someone explain it to me? $||\vec a||$

REPLY [3 votes]: This notation represents the magnitude of whatever is inside, generally a vector. See this.