How early I can apply for US citizenship?

I know I can apply citizenship 5 years after getting green card. But I heard that I can apply earlier to save time.
Can somebody tell me how early I can apply? And why I can apply earlier than 5 years? Are there any difference between different states?
Whoever answer my question, thank you very much.

所有跟帖: 

回复:How early I can apply for US citizenship? -new859- 给 new859 发送悄悄话 (190 bytes) () 07/26/2006 postreply 14:12:17

回复: -YuTu- 给 YuTu 发送悄悄话 (16 bytes) () 07/26/2006 postreply 14:29:04

回复:How early I can apply for US citizenship? -gongxili- 给 gongxili 发送悄悄话 (135 bytes) () 07/26/2006 postreply 18:24:56

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