Let θ be the included angle between A and B, then |a|cosθ is called the projection of vector A in the direction B.

Let θ be the included angle between a and b, then |b|cosθ is called the projection of vector b in direction A.

If two are equal, then |a|cosθ=|b|cosθ.

|a|=|b|, that is, the modulus of A is equal to the modulus of B.