[…] a "chain" is a sequence of categories at S-structure coindexed by Move-α, each member except the first being a trace of the first member, which we will call the "head" of the chain.
- Chomsky (1993), Pag. 331

C = (α1,..., αn) is a "chain" if and only if: (i) α is an NP; (ii) α"i" locally A-BINDS α"i"+1; (iii) for "i" > 1, (a) α"i" is a non-pronominal empty category, or (b) α"i" is A-free; (iv) C is maximal, i.e., is not a proper subsequence of a chain meeting (i-iii).
- Chomsky (1993), Pag. 333