Why do people mix typefaces? (pg54)
The mixing of typefaces is a practice where a person most likely a disigner uses more than one typeface together. The reason for this is to creat something different and becomes sort of a challege. It give contrast to the design. The combining of the typefaces most be handle with care, the values of the different strokes of the typeface most be taking into account as well as the tone of the typefaces. For example Helvetica is neutral typeface while Bodoni has an elegant element, so mixing these two typefaces might not be the best idea. What needs to be taken into consideration also is the text hieght, and the length of its ascendor or descendor.
http://designshack.net/articles/typography/mixing-typefaces-tips-and-techniques/