Basic shape of the ‘a’
First things first, this:
|Linux Libertine Regular|
Parts of an 'a'
This is the annoying part with all the technical terms. There are dozens of names (maybe hundreds—I didn't count them all) to let typographers describe every part of every letter. There are a lot to learn, but the terms can actually be really useful. The basic components of the letter ‘a’ are labelled in this diagram:
|Components of a lowercase letter ‘a’|
Arc of stemThe overhang on top of the ‘a’. It's the main thing that distinguishes ‘a’ from ‘ɑ’.
TerminalThe thing found at the end of the terminal. It can be shaped like a wedge, blob, ball, or simply taper out in some typefaces. It can also have a serif, though then it’s not technically a terminal anymore.
StemThe backbone of the ‘a’, highlighted in pink. Stem always refers to a straight vertical part of a letter.
SpurAn extension of the stem unique to ‘a’. Sometimes it's called a swash or a tail. In some fonts like Libertine or Minion, it takes on its own unique shape, while in others, it might just look like an ordinary half-serif. The tail usually bends away from the letter, even in some sans serif typefaces.
BowlThe loop that makes up the bottom-left part of the ‘a’.
CounterThe hole in the ‘a’ enclosed by the bowl.
Constructing the body of the ‘a’
A big thing that a lot of serif ‘a’s have that you probably never noticed is that their diagonals are largely parallel. Not all serif ‘a’s have this—either because the designer had something else in mind, or, as I suspect in the case of the Droid and DejaVu serif fonts, because the designer just stuck serifs onto a sans typeface. But most old styles and many others have this.
|Left to right: Minion Pro, Linux Libertine, Charter|
The bounds for the bowl and the shoulder of the terminal are somewhat perpendicular to the other lines. Even though the lines aren’t exactly parallel—the diagonal vectors converge at a vanishing point somewhere to the upper right (the way a right-handed person draws near-parallel lines)—it’s still useful for constructing the ‘a’.
|The letter ‘a’ can be roughed out with some lines and boxes.|
The terminal is a small part of the ‘a’, but it's very influential over the character of the letter. An ‘a’ can have a serif, ball terminal, drop terminal, or have no blobbing at all.
This is the terminal I chose for my ‘a’ :
Almost every serif ‘a’ has a spur, but the shape varies. Sometimes it’s just a half-serif, other times it has its own unique shape. Sometimes it’s angled parallel to the bowl and arc of stem, and sometimes it’s flat to the baseline. Even if it’s not exactly a perfect serif, the spur is often derived from a serif, so it’s best to wait until you design a letter like ‘i’ and come up with the serifs before tackling the spur of the ‘a’. Until then, feel free to just put a placeholder there—As you might have noticed, I did something like the spur on the Minion ‘a’.