form

Form is the shape of a thing. Every letter in the alphabet has a separate form. For example, a capital 'H' has straight lines at right angles, while a lower case 'a' contains rounded forms. In design, contrast of form refers to the contrast between different type families. The difference between a roman letter and italic for example, or a capital contrasted with a lowercase or a condensed letter with a compacted one.
  If their forms are too similar there isn't an effective contrast. A script for example would not contrast well with an italic for they are both based on cursive handwriting and share the same characteristic slope.