Contrast is created when you have two or more related elements that are different. Contrast can be used to add visual interest, depth and to highlight a dominant element. It is also a way to unite a piece and often bestows energy to a design. If there are too many contrasts or the differences are too severe, there is risk they will not only compete for attention, but may also create unbalance, confuse the viewer and create optical illusions. To achieve success with contrast, ensure the differences are obvious enough, but without an overpowering presence. Each design element mentioned in this article can be manipulated to create contrast. Each of the images not only relate to the corresponding element but also exhibit contrast.
In the "Getting Started with Woodworking" video, the holes through the 4x4's were drilled from the back. That is, they start on the side opposite the precisely-positioned mark, and drill through to hit it. If they can do this, more power to them, but I can't drill through 3-1/2" of wood to emerge at a precise mark without a drill press - and not always then.
To mark the centerline, set a compass to span something more than half the width of the leg. Draw an arc from corner of the leg. The point where the arcs intersect will be on the centerline. With a centerline point on each end of the leg, place a scribe on the point, slide a straightedge up to touch the scribe. Do the same on the other end. When you have the straightedge positioned so that you can touch both points with the scribe, and in each case it is touching the straightedge - without moving the straightedge - scribe the line. Use scribes, rather than pencils or pens, because they make more precise marks.