(in 3-dimensional space)
A straight line in 3-dimensional space can be specified as the intersection of two planes. Thus a straight line is given, in general, by two linear equations, a1x+b1y+c1z+d1=0 and a2x+b2y+c2z+d2=0. (If these equations represent identical or parallel planes, they do not define a straight line.) Often it is more convenient to obtain parametric equations for the line, which can also be written in ‘symmetric form’. See also vector equation (of a line).