In transformational grammar, a trace is an empty (phonologically null) category that occupies a position in the syntactic structure. In some theories of syntax, traces are used in the account of constructions such as wh-movement and passive. Traces are important theoretical devices in some approaches to syntax.
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Empirical evidence pointing to the existence of traces, independently of all theory-specific considerations, has also been presented in the literature. For example, for many English speakers, the contraction of want to to wanna is possible in some contexts, but not in others:
One way to explain this contrast is to assume that the trace left behind by the extraction of which candidate in the second example blocks the contraction of want and to.
The statement "I want John to go" is transformed into an 'echo'-question. If "who" is moved to the beginning of the sentence, it will leave a trace. The existence of the trace will make it impossible to contract "want" and "to". Consider the following:
Vicky wants that candidate to win. ==> Vicky wants which candidate to win?
^ v
|____________|
==> Which candidate does Vicky want to win? (not "Which candidate does Vicky wanna win?")
However, the validity of this and similar arguments have been called into question by linguists favoring non-transformational approaches.
In government and binding theory, traces are subject to the empty category principle (ECP), which states that all traces must be "properly governed". Proper government is either theta-government or antecedent-government:
However, intermediate traces are not subject to the ECP because they are deleted at LF (logical form).