If two variables, say x and y, are functions of a third variable, say θ, then the functions expressing x and y in terms of θ are called the parametric representations of x and y. The variable θ is called the parameter of the function.

Equation of a straight line passing through the point (x_{1}, y_{1}) and making an angle θ with x-axis.

Any point on this line can be taken as (x_{1} + r cos θ, y_{1 }+ r sin θ) where r is the algebraic distance.