Suffixation is the addition of a bound morpheme at the right edge of a base word. In Frisian, suffixes always determine the syntactic category of the complex word as a whole, and in the case of the formation of nouns, they also determine the gender of the noun. The diminutive suffix -DIM, for example, always creates neuter nouns, independent from the input category.

Suffixes also determine the stress of the formation. Those with a nucleus consisting of a schwa do not bear inherent stress by definition, and therefore leave the main stress on the base. This is even more pregnant in the case of suffixes that are built up with one or two consonantal suffixes. Other suffixes may take over the main stress of the output form, however. An example is -inne, as in boerfarmer > boeRINnefarmer's wife. We see this behaviour with non-native suffixes in particular.

Another feature relevant for phonology is the distinction between cohering and non-cohering suffixes. The distinction pertains to the interface between morphology and phonology: cohering affixes form a prosodic word with their stem, while non-cohering affixes are prosodic words of their own. Thus, the morphological boundary of a cohering affix does not necessarily coincide with a prosodic boundary. All non-native suffixes and most native suffixes are cohering. A number of native suffixes, however, is non-cohering. These are the following suffixes, which all derive historically from words: -eftich, -ber, -dom, -heid, -ling, -leas, -skip, -sum . For the same historical reason, suffixoids are always non-cohering.

Usually, formations derived by suffixation can be the input to further suffixation. This is not the case with closing suffixes. An example is the diminutive suffix -DIM. The possibilities for further word formation are treated in the section "morphological potention" in the topics on the individual suffixes.

The treatment of suffixes has concentrated here on the native stock. Therefore, not all suffixes of non-Germanic origin have been dealt with, but the main ones have been given attention, among them the ones that, mostly exceptionally, may also take native bases. For example, the suffix -emint, from French -ement, may be attached to native words as in preuvelemintmurmuring (from the verb preuveljeto murmur) or kakemintlower and upper jaw (from the noun kaakjaw). Anyhow, it is sometimes hard to establish in how far non-native suffixes have an independent role in the word formation of Frisian. They are often not productive, and in many cases one may assume that the non-native words have been loaned in their entirety, usually via the majority language Dutch.

Suffixation is a very common means to derive nouns and adjectives, and, to a somewhat lesser extent, also verbs. In addition, a number of suffixes can derive adverbs. Compared to these main categories, the derivation of numerals, interjections and names is only marginal.

As a final remark: suffixes are also active in the formation of synthetic compounds. These are not treated here under the heading "suffixation", but rather under "composition".