Morphemic Tier Hypothesis

The Morphemic Tier Hypothesis (MTH) is a hypothesis first introduced into the theory of Autosegmental phonology in McCarthy (1981) which entails the claim that every morpheme making up a word is assigned a separate tier, i.e., a separate and autonomous level of representation. This hypothesis is mainly proposed to circumvent the No-Crossing Constraint which says that association lines may not cross.

Example
the Arabic word katab is made out of the triliteral root ktb 'write', the perfective active morpheme a, and the template CVCVC. If the morphemes ktb and a were represented at a single tier, association of these morphemes to the template CVCVC would result in a violation of the No-Crossing Constraint, as is shown in (i). By representing them at autonomous tiers as in (ii), this problem is solved:

(i) *	C V C V	C	* C V C V C        | | |   |         |   |   | k a t	b	 k   t   b  a (ii)	    a            / \ C V C V	C |  |   | 	k   t	b

Links
Utrecht Lexicon of Linguistics