Valency of Word-Building Elements.
It is one of the most essential characteristics of word-building. Valency is defined as the ability of a linguistic unit to be combined with other linguistic units of the same order. Here we can speak of the valency of affixes and stems.
The most important feature of word-building elements is their ability to be combined with each other from the point of view of their part of speech characteristics. So, the noun-forming suffixes -ship, -hood, -let can be added to noun-stems (e.g., friendship, boyhood, eyelet), and the suffixes -ment, -tion, -ing, -ее, -ence/-ance are added to verbal suffixes (e.g., enjoyment, introduction, reading, evacuee, assistance). The suffix -er is easily combined with verbal suffixes (runner, fitter, etc.), but occasionally it can go with noun suffixes (e.g., astronomer, lifer). The valency of some of the adjective-forming suffixes is as follows: the suffix -able is added to verbal stems (regrettable), -ful – to noun stems with an abstract meaning (respectful, beautiful), -ish can go with noun stems (boyish) and with adjective stems (greenish). Note that the suffix -ful added to noun stems with the meaning of a container gives a noun (roomful, spoonful).
There are certain constraints (restrictions) on the ability of the elements to be combined with each other. They can be phonetic, morphological, syntagmatic and semantic. Some morpho-phonological constraints can be illustrated by the suffix -tion/ -ion/-sion: the allomorph -tion can't follow the consonant -n-. Cf: unite - union (not unition), opine - opinion (not opintion).There are some syntagmatic constraints on valency: dis- can only precede the stem, being a prefix, but it can never follow the stem, e.g., discharge, but not charge-dis. Some semantic constraints: the meaning of the stem must agree with the meaning of the affix. For example, adjectives can be formed by adding the suffix -ful to noun stems denoting quality or state but not to stems denoting objects, e.g., sorrowful, beautiful, joyful, but not boyful. Some morphological constraints: affixes can usually be attached to certain morphological classes of stems, e.g., -less can be added to noun stems, but not to verbal or adjective stems (e.g., motionless, but not moveless); -ing, -able go with verbal stems, but not with noun or adjective stems (e.g., readable, building), -ish can be added both to noun or adjective stems (e.g., greenish, boyish).
Valency can be broad or narrow. For example, the noun-forming suffixes -er, -ing can be added to unlimited numbers of verbal stems, e.g., netting scribbler, shuffler, running. The noun-forming suffix -ful can be added to any nominal stem meaning «container», e.g., spoonful, mouthful, roomful, etc., and the suffix -less – to nominal stems denoting some property, e.g., voiceless, bottomless, even typewriterless.
On the contrary, the suffix -ledge can be combined with a very limited set of stems, e.g., knowledge, but not thinkledge.
Some constraints are imposed by linguistic norm, by usage, though it is difficult to explain their reasons. For example, it is difficult to explain the rule choosing the prefix un- or in-, e.g., inadequate, but unattractive; unimportant, but impossible; also, unjust, but injustice, though unfaithful and unfaithfulness. The last case is rather regular, but unaccountable.
It is also difficult to explain some variant forms, which are possible in some cases but impossible in others, e.g., debarkation and debarkment are variants, but demarcation has no variant like debarkment.