Just came across the Nested Dependency Condition - if two wh-trace dependencies overlap, one must contain the other. What does this mean?
It's about dependencies between fillers and gaps; in English, you can say, This is the knife[k] that this salami[s] is easy to cut __[s] with __[k]
There are two dependencies, one between the knife and the gap marked with k, and one between the salami and the gap marked with s. The s dependency is contained within the k dependency, so they stand in a “nesting” relation.
Compare the following, with crossing dependencies: *This is the salami[s] that my knife[k] is easy to cut __[s] with __[k].
So the Nested Dependency Condition requires that multiple dependencies nest each other, in English. The examples are from Kuno and Robinson, 1972, "Multiple wh questions," in Linguistic Inquiry.