Before you can understand lettering, it's best to become acquainted with typography — the art of how letters are formed and arranged in text. Typography is based largely on the interaction between the shapes of letters. When typography is done correctly, it's transparent — the letters form words so coherently that the play between letters themselves is hard to spot.
Serif and Sans Serif
Fonts (also referred to as type) are classified into two categories: serif and sans serif. Serifs are the little “handles” added to the ends of letters; these handles make reading easier by improving the eye's ability to connect letters into words. Sans serif fonts feature letters drawn with simple strokes and no affectation (sans is French for “without”). On your computer, the Times font is serif; Helvetica is sans serif.
The font on top is serif; the one below is sans serif.
One would assume that serif fonts would be the ideal choice for comic lettering, but just the opposite is true. Serif lettering, although easily readable at small sizes, conveys a sense of formality and seriousness. Most serif fonts are completely inappropriate for cartoon usage — except in the special cases, such as captions under some single-panel comics.
The exception to the sans serif rule in cartooning is the capital letter I. It is preferred to have serifs on the I, but only when I is used as a proper noun (I). When it's used as part of another word (“It,” “Idea”), the I should be typeset sans serif.
Rules of Alignment
You have three options for aligning your text. You can center it, or you can justify it to the left or right margin (“flush left” and “flush right,” respectively). Most typography in comic strips and books is centered (this works especially well in word balloons), which means that the line of type is centered between the left and right borders. Beginners should lightly pencil the letters in blue first, adjust to center the line of text, and then pencil in the lettering more firmly.
Centered, flush-left, and flush-right text