A set of rules determined by convention for giving plus or minus signs to distances in the formulae involving lenses and mirrors. The real-is-positive convention is now usually adopted. The New Cartesian convention is now not widely used. In this convention distances to the right of the pole are treated as positive, those to the left as negative. This system has the advantage of conforming to the sign convention used with Cartesian coordinates in mathematics and is therefore preferred by some for the more complicated calculations.