addressing invites to people in the military

we have guests - a couple - one is a Major in the Marines and one is a Lieutenant Commander (I think) in the Navy. He's the major, she's the Lieut.

Does anyone know the proper way to address the envelopes?

Frankie cousin is a major so we addressed it as

Major & Mrs. Steven Smith

I was just reading an article in one of the bride magazines all about how to address professionals and people in the military. I was actually surprised about how you should adress them.
I will look for it this weekend and let you know which magazine it was in....

Thank you!

I'm going to call the Major's sister..she will know too I'm sure. It's more how to address it since both are in the military...so I can't do Maj. and Mrs...

Thanks..