A plot of 206Pb:238U against 207Pb:235U for concordant samples of various ages should define a single curve, named ‘Concordia’ by G. W. Wetherill (1956). If the ratios plotted for samples fall below this concordant age pattern expected for the rock body, this produces a discordant age pattern. A straight line drawn through two or more such points should intersect the Concordia curve at two points. One of these will give the true age of the rock, while the younger intersect should give the date of any lead loss (which is what causes the discrepancy).