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SYNTAX: 'Look-ahead' is an intuitive term for the phenomenon that certain grammatical principles filter out derivations which under local economy can only be filtered out by looking ahead to a later stage in the derivation.
Instances of look-ahead in Chomsky (1995) are:
- determining which features are uninterpretable at the interfaces and hence must be deleted (by either movement or merger of an element with matching features)
- determining which element projects when two elements are merged
- global economy principles like Procrastinate
Chomsky (1998) tries to do away with look-ahead by applying Spell-Out and LF-interpretation cyclically to phases. EXAMPLE: In general, Procrastinate bars overt movement to weak checking positions. In some cases, however, Procrastinate can be violated:

(i) What did you see?
In (i) what is moved from a weak position (case in vP) to a strong position (Q in CP). Overt movement to weak checking positions can only occur when it is followed by movement to a strong checking position, and so it seems like you have to look ahead in the derivation to determine whether overt movement to a weak position is allowed.