The first time I quilted piano keys on a border a couple of years ago, I didn't know nearly what I do now. I was just beginning to experiment with ruler quilting on my HQ Sweet16. I was adding the piano keys outside a feathered border, filling the space between the feathers and the edge of the quilt, but only used the edge of the feathers and the edge of the border as guides. I believe I used the lines etched in my ruler as guides, but nothing to help me stay square with the quilt itself. My lines often leaned one way or the other, sometimes quite obviously. I entered that quilt in a county fair and remember the judge commenting that the piano key lines needed to stay straight. Lesson learned.
Since then, I have added piano keys or beadboard to several quilt borders of my own and to customer quilts with great success. Although I had quilted both, I learned only recently the difference: Piano Key are single lines spaced evenly like the keys on a piano. Beadboard are double lines placed close together (mine are 1/4" apart) and the doubles spaced about an inch apart, similar to beadboard panels used in construction. The quilt pictured below had several smaller borders and a larger outer border. I treated each inner border separately and then quilted the beadboard in the final outer border.
This quilt had a double outer border that I treated as one with beadboard quilted from the black inner border to the edge of the quilt.