A.Sal Alfano responds: Follow this procedure to lay out shingle siding so the courses break on the same line as the window trim: First, measure the window height and add the width of any trim you might have, then divide by the shingle reveal. Most of the time, you won’t have an even number of courses, and you’ll have to adjust the shingle reveal slightly. To do that, ignore the fraction and choose the closest whole number; this will be the number of even courses that will fit between the window trim. Then pull your tape measure out to an even multiple of that number of courses, run it at a diagonal next to your window (see Figure 1), and tick off each multiple. These marks will be your course lines.

Let’s take an example: Say your window height, including the head trim and apron, is 49 inches and your average shingle reveal is 8 inches. Since 49 ÷ 8 = 6 1/8, you will have six courses (ignoring the fraction). To find the exact width of each reveal, pull the tape out to some large multiple of six, say 60 inches, and run it at a diagonal from...