Relative clauses headed by zeros (represented as Ø in the examples below) are sometimes called contact clauses or contact relatives.
- Contact Clause
- Garden-Path Sentence
- What Are Grammatical Zeros and Bare Relatives?
- Who, Which, and That
Examples and Observations:
- The house Ø I bought last year had sustained some fire damage.
- The woman Ø I hired to look after my mother is wonderful.
- I disagreed with most of the points Ø she raised.
- The book Ø he selected was Walden.
- When to Use the Zero Relative Pronoun
"On occasion, we can correctly omit the relative pronoun from a relative clause. The gap left by the omitted pronoun is called a zero relative pronoun. If the omission does not bring a verb to the head of the relative clause, it is perfectly correct to remove the relative pronoun. The sentence will make complete sense without it.
The car (that) we saw yesterday was too expensive.In each example, the omitted relative pronoun is in parenthesis because it is optional. In the first example, the relative clause we saw yesterday modifies the noun car. We could write the clause with the relative pronoun that included, but we do not have to. In the second example, the relative clause we know modifies the noun people. We could have included the relative pronoun whom in the clause, but the sentence makes perfect sense without it.
The people (whom) we know are not very responsible.
"In other sentences, removing the relative pronoun would make a verb the first word in the clause and cause the sentence to be grammatically incomplete.
The men who repaired our roof did a wonderful job. (correct)Try leaving off the relative pronoun in each example.
We all saw the show that won the Tony Award this year. (correct)
The men repaired our roof did a wonderful job. (incorrect)These sentences do not amount to much. When appropriate, feel free to use a relative clause containing a zero relative pronoun. Just be sure that your sentence still makes sense."
We all saw the show won the Tony Award this year. (incorrect)
(M. Strumpf and A. Douglas, The Grammar Bible. Owl Books, 2004)
- The Zero Relative Pronoun and Syntactic Ambiguity
"[I]f a zero relative pronoun is used, it may be possible for the first word of the relative clause to be interpreted as part of the main clause; Temperley  gives the example phrase the biological toll logging can take, where the first four words are ambiguous on an initial reading--logging may be the head noun of the NP or the subject of the upcoming relative clause--the ambiguity only being resolved on the word can, which as a modal verb indicates that the word before it is more likely to have been a subject."
(Tony McEnery and Andrew Hardie, Corpus Linguistics: Method, Theory and Practice. Cambridge Univ. Press, 2012)