Let's take a closer look at behavior of these two classes of morphemes. Roots also contribute the greatest conceptual content to the overall meaning of the word. Since roots are doing most of the work of conveying meaning, they are required word elements. For example, pter is a root meaning 'wing'. Also, the meaning 'wing' is a vital and central part of the overall meaning of the word.
Affixes are morphemes which attach to roots or a combination of roots and other affixes.
Their main use is to modify the meaning conveyed by the root or roots. Consider the affix pro - meaning 'before, for, forward'. Although the different senses of pro- are clearly related, the exact meaning shifts somewhat from word to word. As a rule, affix meanings tend to be vaguer and more variable than root meanings. Look back at the examples we just discussed.