The following are the general forms of agreement, sometimes called the Four Concords: -
1. The agreement of the Noun in Apposition or as Predicate (§§281 - 284).
2. The agreement of the Adjective with its Noun (§ 286).
3. The agreement of the Relative with its Antecedent (§ 305).
4. The agreement of the Finite Verb with its Subject (§ 316).
a. A word sometimes takes the gender or number, not of the word with which it should regularly agree, but of some other word implied in that word.
This use is called Synesis, or cónstrúctió ad sénsum, (construction according to sense).