A term used to refer to the proper positioning of all typefaces and size variations along an imaginary reference line. Alignment is a necessary consideration that ensures that all styles and sizes can be mixed in the same line.
Base alignment (or horizontal alignment) involves aligning the bottom of each character along a horizontal baseline. Vertical alignment involves alignment of characters beneath each other along an imaginary vertical line to the left of the characters.
Optical alignment involves the use of some visual reference point. For instance, in the setting of vertical lines, characters would not be flush left but centered. In some cases, optical alignment may require that curved or angular characters (such as "O"s) be slightly off the baseline to achieve visual alignment. Although they may be geometrically correct, they simply will not "look right" to the eye.