Either 可當限定詞、代名詞、副詞或連接詞用。這個字其實有兩種發音，國內大多教美式發音 /ˈi:ðə(r)/，但英式英語亦發成 /ˈaɪðə(r)/。
Either 用作限定詞、位在名詞之前時意為「兩者之中任何一個的；兩者的」。在此用法中，either 後面的名詞必須是單數可數名詞 (雖然 either + 單數可數名詞可用 both + 複數可數名詞來替代，但 both 只能用在肯定句)。例如：
- I don’t like either color. (這兩種顏色 [中任何一種] 我都不喜歡) (正)
I don’t like either colors. (誤)
I don’t like both colors. (誤) - John studied chemistry and physics, but he had little interest in either subject. (約翰唸的是化學和物理，但他對這兩門學科都沒什麼興趣) (正)
John studied chemistry and physics, but he had little interest in either subjects. (誤)
John studied chemistry and physics, but he had little interest in both subjects. (誤) － little 為否定詞。 - Either candidate would be ideal for the job. (這兩個人中的任何一個都是做這項工作的理想人選) (正)
Either candidates would be ideal for the job. (誤)
Both candidates would be ideal for the job. (正) - There are three cars on either side of the river. (河的兩邊各有三輛汽車 － 總共有六輛車子) (正)
= There are three cars on each side of the river. (正)
= There are three cars on both sides of the river. (正)