Relative clauses & subject-verb agreement

In a sentence, there must also be agreement between the verb form used in a relative clause and the noun which the relative clause is adding information to; for example:

In the sedimentation tank some grease rises to the surface as scum, which is scraped off.

The highlighted clause is called a relative clause. Which refers to scum, a singular noun, in the previous clause, so the verb following which must also be singular in number.

Here is another example:

The waterfront workers, who were on strike, refused to sign individual employment contracts.

In this example, who refers to the workers, a plural noun, in the previous clause, so the verb following must also be plural in number.