Classification of Type

Old Style: 1475
-Wedge Shaped Serifs
-More Upright Stress
-Horizontal Crossbar
-Greater Thick-Thin Contrast
Examples: Bembo, Garamond, Plantino, Sabon, Albertina

Transitional: 1750
-More severe, business like, and sharper
-First types that were really designed
-Axis of thick-thin contrast is almost vertical or slopes slightly left
-Bare serifs only a little or not rounded at bottom
-Top Serifs of lower-case letters are roof shaped
Examples: Baskerville, Concorde, Fournier, Perpetua, Caslon Old Face

Modern: 1775
-Strong emphasis on vertical stroke
-Sharp Contrasts
-Symmetry and sharp transition to the straight serifs
-Serifs are as thin as thin parts of the letter
Examples: Didot, Bodoni, Walbaum, Linotype Centennial, Walbaun

Slab-Serif: 1800's
-Only a slight thick-thin contrast
-Heavy rectangular serifs are as thick as the letters
-Serifs are the defining characteristic
-Differences in subdivision most visible in lowercase letters
Examples: Antique, First Clarendon, Beton, Memphis, Serifa

Sans-Serif: 1920's
Humanist 
-No Serifs
-Line widths visually equal
-More distinguishing forms than other sans-serifs
Examples: Gill Sans, Profile, Frutiger, Scala Sans, Myriad

Grotesk (and Gothic)
-Axis of the rounding is vertical
-Ascender height is usually equal to the capital height
-Narrow, vertical, construction of the letters
-Capital Angular
Examples: Akzidenz Grotesk, Helvetica, Franklin Gothic, Vectora, Bell Gothic

Geometric
-Line thicknesses only visually and minimally corrected
-Axis of the rounding is vertical
-Letters seem to be drawn using ruler and compass
Examples: Futura, Avant Garde, Eurostile, Erbar, Nevzeit Grotesk