Approach used to visualize or quantify joint action of chemical mixtures by plotting a graph (isobologram) of equally effective dose pairs (isoboles) for a single effect level. A particular effect level is selected, such as \(\pu{50\%}\) of the maximum, and doses of drugs \(\ce{A}\) and \(\ce{B}\) (each alone) that give this effect are plotted as axial points in a Cartesian plot. The straight line connecting \(\ce{A}\) and \(\ce{B}\) is the locus of points (dose pairs) that will produce this effect in a simply additive combination. This line of additivity allows a comparison with the actual dose pair that produces this effect level experimentally.