Adjacency Condition

Adjacency Condition is a condition on word formation rules proposed in Siegel (1977) and taken up by Allen (1978) which states that an affixation rule can be made sensitive to the content of an embedded morpheme only if that morpheme is the one most recently attached by a morphological rule. Intuitively, the Adjacency Condition prevents a word formation rule from looking into the entire derivational history of morphologically complex words.

Examples
In Dutch, adjectives can be formed by adding the suffix -baar '-able' to verbal stems. However, this suffix takes transitive verbs as its input only. Hence, vinden 'to find' vindbaar 'findable', but klimmen 'to climb' *klimbaar 'climbable'. When the transitivising prefix be- is added to the intransitive verb klimmen though, the derived transitive verb beklimmen 'climb' may undergo -baar suffixation (beklimbaar 'climbable').

The Adjacency Condition accounts for the contrast between *klimbaar and beklimbaar in the following way. In the first example, the content of the verb klimmen is structurally adjacent to the suffix -baar (klim]V baar]A), and given the condition on -baar [[suffixation, the adjective klimbaar will be ruled out. The situation is crucially different in the second example. In this form the content of the verb klimmen is not structurally adjacent to the suffix -baar (be [klim]V]V baar]A), and the Adjacency Condition makes the -baar suffixation rule insensitive to the intransitive nature of the [[verb klimmen.

Comment
Williams (1981) has replaced this condition with his slightly different Atom Condition.

Link
Utrecht Lexicon of Linguistics