The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
306. A Relative generally agrees in gender and
number with appositive or predicate noun in its own clause rather than
with an antecedent of different gender or number (cf. § 296. a): -
- mare etiam quem Neptúnum
esse dícébás (N. D. iii. 52), the sea, too,
which you said was Neptune. [Not quod.]
- Thébae ipsae, quod
Boeótiae caput est (Liv. xlii. 44), even Thebes, which
the chief city of Boeotia. [Not quae.]
NOTE: This rule is occasionally violated: as, - flúmen quod appellatur Tamesis
(B. G. v. 11), a river which is called the Thames.
a. A relative occasionally agrees with its
antecedent in case (by attraction): -
- sí aliquid agás
eórum quórum cónsuéstí
(Fam. v. 14), if you should do something of what you are used to
do. [For eórum quae.]
NOTE: Occasionally the antecedent is attracied into the case
of the relative: -
- urbem quam statuó vestra est (Aen. i. 573), the
city which I am founding is yours.
- Naucratem, quem conveníre voluí, in naví
nón erat (Pl. Am. 1009), Naucrates, whom I wished to
meet, was not on board the ship.
b. A relative may agree in gender and
number with an implied antecedent: -
- quártum genus ...
quí in vetere aere aliénó vacillant
(Cat. ii. 21), a fourth class, who are staggering under old debts.
- únus ex eó numeró
quí parátí eraut (Iug. 35), one of
the number [of those] who were ready.
paucí, dé quá [i.e. coniuratione]
dicam (Sall. Cat. 18), a few have
conspired, of which [conspiracy] I will speak.
NOTE: So regularly when the antecedent is implied in a
possessive pronoun: as, - nostra ácta,
quós tyrannós vocás (Vat. 29), the
deeds of us, whom you call tyrants. [Here quós agrees with the nostrum (genitive plural) implied in nostra.]