Given a line in 3-dimensional space, let (x1,y1,z1) be coordinates of a point on the line, and l, m, n be direction ratios of a direction along the line. Then the line consists of all points P whose coordinates (x,y,z) are given by
for some value of the parameter t. These are parametric equations for the line. They are most easily established by using the vector equation of the line and taking components. If none of l, m, n is zero, the equations can be written
which can be considered to be another form of the parametric equations, or called the equations of the line in ‘symmetric form’. If, say, n = 0 and l and m are both non-zero, the equations are written
if, say, m = n = 0, they become y = y1, z = z1.
More generally in n-dimensional space, if p and a are in ℝn with a ≠ 0, then
is a parameterization of the line passing through p which is parallel to a.