Promise Ring

Yesterday was my birthday and my boyfriend, of 3 years, gave me this beautiful ring. I read that you put the promise ring on your ring finger, but im not sure if that was his intensions. I dont even know if he knows what a promise ring is. How do i ask him about it without making him feel pressured? or should i just put it on a different finger? i dont know what to do... Please help, im going to get it sized soon so i need to know asap. Thanks sooo much!!!! anything will help.