Plot of Ca ii line strength (W₈₅₄₂ + W₈₆₆₂) against magnitude difference from the horizontal branch (V − V_HB) for the standard clusters. In order of increasing W₈₅₄₂ + W₈₆₆₂ values at V − V_HB = –1.5, the solid lines are for clusters M 15 (individual stars plotted as open 6-pt star symbols), NGC 4590 (open triangles), NGC 4372 (filled triangles), NGC 6397 (open 5-pt stars), M 10 (open circles), NGC 6752 (filled 5-pt stars), NGC 3201 (filled diamonds), M 4 ( × -symbols), M 5 (plus symbols), NGC 6171 (filled squares), M 71 (open diamonds), NGC 5927 (open squares), NGC 6528 (6-pt star symbols), and NGC 6553 (filled circles). The data for each cluster has been fit with a line of slope  − 0.627   Å/mag for V − V_HB ≤ 0.2. Vertical bars on each point show the measurement uncertainty in the line strengths.