既…又 / 既…也【jì…yòu / jì…yě】

既…又 and 既…也 say that one subject has two qualities or does two things: "both … and". 既 goes before the first, and 又 or 也 before the second. Both halves share the subject, which comes before 既.

又 or 也

With 又 the two halves are equal partners, often two adjectives packed close together. With 也 the second half is added on top of the first, and it is the usual choice when the halves are longer phrases or a recommendation: "you need X, and you also need Y".

Neither … nor

With 不 in both halves the frame means "neither … nor".

One subject, before 既

既 is an adverb, so it comes after the subject and can't take a subject of its own in each half. Two adjectives or verbs are joined with this frame, not with 和.

• × 既他很聪明，又他很努力 → 他既聪明又努力
• × 这个菜好吃和便宜 → 这个菜既好吃又便宜

又…又 means much the same and is more casual. 既然 "since" is a different word that happens to start with 既.