All four letters can be related to each other. The letter ‘p’ is traditionally thought of as a letter ‘b’ with its ascender depressed, and given a descender. The ‘d’ in turn is made by rotating the ‘p’ 180 degrees, and replacing its bilateral serif with an angled head serif. The ‘q’ is then made by removing the ascender from the ‘d’ and adding a descender, much like making the ‘p’ from the ‘b’. The descenders are almost always the same length as the ascenders. It's also possible to derive all the letters from one of the four letters—the ‘b’-centric method is shown below.
|The lowercase ‘p’, ‘d’, and ‘q’, related to the ‘b’|