I learned that a two-by-four piece of lumber is not exactly 2 inches x 4 inches (instead, 1.5″ x 3.5″).  The marketed wood item is actually physically smaller in both dimensions:  width and depth.

Today, I learned that a marketed 3-inch diameter plug for a PVC cleanout access is actually about 3 1/2 inches in diameter (possibly because it includes the thread depth of the plug itself).  When I shopped for the item, I could only find 3 inch and 4 inch plugs, no 1/2 sizes!

A 3-inch PVC plug actually fits a tube of larger diameter:  as measured by me, the tube size is 4″ but the plug that fit the cleanout tube was about 3.5 inches, which was listed as a 3″ plug when purchased.  The PVC part is actually larger than the description on the product part.

Pipe diameter versus tube diameter video link