Why is the ninth month called September?
late 14c., from Late Latin corollarium "a deduction, consequence," from Latin corollarium, originally "money paid for a garland," hence "gift, gratuity, something extra;" and in logic, "a proposition proved from another that has been proved." From corolla "small garland," diminutive of corona "crown" (see crown (n.)).
A statement that follows with little or no proof required from an already proven statement. For example, it is a theorem in geometry that the angles opposite two congruent sides of a triangle are also congruent. A corollary to that statement is that an equilateral triangle is also equiangular.