In simple terms, suffixation refers to the process of attaching morphemes to stems. Most English speakers are familiar with this concept (or at least with the process). Some examples of suffixation in English include attaching -ing or -ed to verbs to express tense (e.g. baking and baked) and attaching -s/-es to nouns to form plurals (e.g. books). Except for a handful of inflectional ones, great majority of suffixes in English are derivational in their nature. In contrast, suffixation is quite ubiquitous in Turkish both in inflectional or derivational domains. Indeed, it is a very productive and a defining feature of the language. Almost all word classes in Turkish can take on one or more suffixes.
Notwithstanding few exceptions, majority of these suffixes will have two or four variants. That is, each suffix will have two or four different versions, each version made up of a different vowel. The correct variant (version) of the suffix is determined by the features of the last vowel in the stem (for a review see Vowel Harmony).
Two Variant Suffixes
These suffixes only consist of the open unrounded vowels, A and E. The backness of the last vowel in the stem determines the appropriate variant. In other words, when using these suffixes, only the violations of fronting vowel harmony are taken into consideration. Here are some examples of this type:
Four Variant Suffixes
These suffixes have variants that include all close (high) vowels in Turkish (İ, Ü, I and U). Hence, both the backness and rounding of the last vowel determines the appropriation variant. Here is an example of this type using the -ci suffix: