Another isotope of strontium, almost always Sr-86, is used as a reference isotope. Measured Sr-87 to Sr-86 ratios plotted against Rb-87 to Sr-86 ratios yield a straight line, called an isochron, for a suite of samples. The initial Sr-87 to Sr-86 ratio is estimated from the y intercept, and the apparent age is estimated from the slope of the isochron line.

______________________________________________________