What Hand Do You Wear A Promise Ring On

My boyfriend and I have be going out for over a year now and he recently brought me a promise ring. I was wearing it on my left ring finger, but a girl at work said "I thought you were supposed to wear promise rings on your right ring finger." Now I`m all confused as to which finger you wear it on. I mean I thought since your engagement ring goes on your left ring finger a promise ring does to, because a promise ring is a pre-engagement.