When I began to teach myself about mouldings, I didn't even know the shapes had names. One of the first pieces of moulding I learned about is the piece that goes between the masonry of a fireplace and the wood mantle, called a 'bead'.
In the upper right drawing, the bead is the raised hump in the moulding. When used next to a fireplace in this way, the wood on the left side of the bead (see the drawing on the upper left) would be cut off at the far edge of the quirk (the narrow groove carved into the moulding), leaving the moulding to end with the bead. The bead can then be scribed on the back so it fits the irregularities of the brick or stone and sits cleanly against the wood. It is a magical piece because its quirk, maybe as small as 1/16 inch, makes a shadow so that you cannot 'read' the inevitable uneven plane between the masonry and the wood. After I learned about beads and quirks, I saw them everywhere, being used to make joints visually neat and graceful.
The tongue and groove wood paneling system, used extensively in Victorian times, was an excellent surface covering for places that might be damp: bathroom walls, porch ceilings; or banged into: halls, kitchens, school rooms. And in order to make the joint of the pieces less visible, a bead was cut on one edge, and then a bead strip, or two, run down the middle of the panel. When the boards were fitted together, the strips (actually, their shadows) were what caught the eye, not the - possibly uneven - joints. When I realized the trick of the quirk and bead, I was in awe of those who figured it out - what a simple and neat solution!
I discovered that 'quirks' and 'beads' changed size over the years. The depth and width of the channel and the shape of the curve can date a bead moulding and whatever it attaches to. Arts and Crafts quirks and beads are wider and deeper than their Victorian antecedents. Victorian ones are bolder than those cut before the Industrial Revolution. Next time you are in a big box home improvement store, look at the beadboard paneling offered for sale. The quirk is so shallow that a shadow hardly exists. (I can't resist adding: a shadow of its former self.)
Asher Benjamin wrote eloquently about shadow and mouldings, but I skimmed over those plates and discussion with little comprehension until I understood about beads.