Coverline: A line drawn 0.1° F / 0.05° C above the highest of the previous six temperatures on a fertility chart. When three successive temperatures are above this line, a thermal shift is identified and there's a good chance that ovulation has already occurred. Note that a Normal setting for the TCOYF rule interpretation option within the Calculation tab of Preferences window will permit you to have as little as four temperatures when calculating a coverline (but will use six if they are available). A Strict setting requires six temperatures. TCOYF automatically draws a coverline for you on your Chart when you have enough data to do so. The coverline may change as you cycle progresses.
Refer to page 77 in Taking Charge of Your Fertility (Revised Edition) for more information on how a coverline is drawn (in the First Edition, please refer to page 73).