A quantity in which both the magnitude and the direction must be stated (compare scalar quantity). Force, velocity, and field strength are examples of vector quantities. Note that distance and speed are scalar quantities, whereas displacement and velocity are vector quantities. Vector quantities must be treated by vector algebra; for example, the resultant of two vectors may be found by a parallelogram of vectors. A (three-dimensional) vector V may be written in terms of components V1, V2, and V3 along the x, y, and z axes (say) as V1i + V2j + V3k where i, j, and k are unit vectors (i.e. vectors of unit length) along the x, y, and z axes. See also triangle of vectors.