The language MIX consists of all strings over the three-letter alphabet {a, b, c} that contain an equal number of occurrences of each letter. We prove Joshi’s (1985) conjecture that MIX is not a tree-adjoining language. 1 Introduction The language MIX = { w ∈ {a, b, c}∗ | |w|a = |w|b = |w|c } has attracted considerable attention in computational linguistics.