roots and affixes

Roots vs. affixes

Morphemes (minimal units of meaning) are of two basic kinds: roots and affixes. While there is not an absolutely sharp dividing line between them, due to the natural, gradual historical progression from root to affix, there are various properties that typically cluster together, thus allowing us to distinguish the two types. For most morphemes, it is clear which class they belong in.

Properties of roots:

main part of word

must be at least one in a word

in English, limited to two in a word (simple words have one, compound words have two); where roots are bound, as in Latin or Greek, more can occur in a word, but the number of roots in a particular word is generally small;

can occur independently (free roots)--although bound roots, particularly classical, occur

tend to have richer, more specific semantic content

position is relatively free with respect to other roots (cf. photograph vs. telephoto)

Properties of affixes:

subordinate part of word

not necessarily present--some words occur without any

multiple affixes can occur in a word (e.g. in-divis-abil-ity)

are dependent (bound) elements (where independent form found, generally to some degree dissociated from the bound version)

have more "schematic" (non-specific) content; often grammar-like function

can either precede or follow their roots (prefixes and suffixes,respectively)

position for a given affix with respect to root is fixed

A third type of linguistic element is a function word, which occurs in certain languages like English, which don't have much bound morphology -- that is, languages with lots of free morphemes, instead of mostly words with roots and attached bound morphemes.
In such languages, many grammatical functions are served by function words: small units that have some independence, occuring with more freedom of position than affixes (thus they are somewhat root-like), but which have grammar-like meaning rather than concrete lexical content (which makes them more affix-like). Some function words in English are the, a, he, she, it, if, although, etc.
Function words can be thought of as right in between roots and affixes. Prepositions (like English over, in, through) are sometimes classed as function words and sometimes as roots--because they are, again, intermediate. In form, they are free morphemes. In terms of function, they have (especially in their spatial meanings) more concrete lexical content than most grammatical elements, but their meaning is still rather abstract and relational. (Note that in Greek and Latin, the elements corresponding to the English prepositions are bound morphemes rather than free function words. These are the spatial prefixes such as circum-, meta-, sub-, etc.)

ROOT > PREPOSITION > FUNCTION WORD > AFFIX

The elements to the left are more lexical, those to the right are more grammatical.
Fortunately, we don't have to worry about the in-between cases! Probably 99% of all morphemes in English (and Latin and Greek for that matter) are easily identified by the criteria given above as either roots or affixes. Be sure you can tell the difference when looking at the morphemes in ordinary, garden-variety words.