In theoretical linguistics, a distinction is made between endocentric and exocentric constructions. The distinction between "endocentric" digraphs and empty letters is somewhat arbitrary. Both endocentric and exocentric compounds have been described for a variety of sign languages. The two dependency trees show the manner in which dependency-based structures are inherently endocentric. The subsequent chapter on nominal lexicogenesis discusses endocentric derivation, nominalization and nominal composition. The Coordinative, Verb-Complement, and Endocentric types are also known as Parallel, Verb-Resultative, and Modifier-Head, respectively. A phrase lacking a head is known as exocentric, and phrases with heads are endocentric. In other words, dependency-based structures are necessarily endocentric, i.e. they are necessarily headed structures.