the definitions of the other three units: String Any single element or combination of elements that are continuous in the horizontal dimension (x-axis). Catena Any single element or combination of elements that are continuous in the vertical dimension (y-axis). Component Any single element or combination of elements that form both a string and a catena. Constituent A component that is complete. A component is complete if it includes all the elements that its root node dominates. The string and catena complement each other in an obvious way, and the definition of the constituent is essentially the same as one finds in most theories of syntax, where a constituent is understood to consist of any node plus all the nodes that that node dominates. These definitions will now be illustrated with the help of the following dependency tree. The capital letters serve to abbreviate the words: All of the distinct strings, catenae, components, and constituents in this tree are listed here: Distinct strings A, B, C, D, E, F, AB, BC, CD, DE, EF, ABC, BCD, CDE, DEF, ABCD, BCDE, CDEF, ABCDE, BCDEF, and ABCDEF. Distinct catenae A, B, C, D, E, F, AB, BC, CF, DF, EF, ABC, BCF, CDF, CEF, DEF, ABCF, BCDF, BCEF, CDEF, ABCDF, ABCEF, BCDEF, and ABCDEF. Distinct components A, B, C, D, E, F, AB, BC, EF, ABC, DEF, CDEF, BCDEF, and ABCDEF. Distinct constituents A, D, E, AB, DEF, and ABCDEF. Noteworthy is the fact that the tree contains 39 distinct word combinations that are not catenae, e.g. AC, BD, CE, BCE, ADF, ABEF, ABDEF, etc. Observe as well that there are a mere six constituents, but 24 catenae. There are therefore four times more catenae in the tree than there are constituents. The inclusivity and flexibility of the catena unit becomes apparent. The following Venn diagram provides an overview of how the four units relate to each other: History The catena concept has been present in linguistics for a few decades. In the 1970s, the German dependency grammarian Jürgen Kunze called the unit a Teilbaum 'subtree'. In the early 1990s, the psycholinguists Martin Pickering and Guy Barry acknowledged the catena unit, calling it a dependency constituent. However, the catena concept did not generate much interest among linguists until William O'Grady observed in his 1998 article that the words that form idioms are stored as catenae in the lexicon. O'Grady called the relevant syntactic