Two, unless the vectors are colinear.

You may recall from high school geometry that two lines (not colinear) define a plane. This is the same thing. Most of the examples in this chapter use 2D vectors. But you can think of them as 3D vectors that lie in a plane.

The figure shows a vector **w** and an arbitrary vector **v**.
For convenience in visualization,
their tails start from the same point
(but, of course, vectors have no position).

Also for convenience, the vector **v** is horizontal;
but it could be at any orientation.