When did phone numbers go from 6 digits to 7? PHONE-TARDS NEEDED.

If i am not mistaken, a long time ago, phone numbers had only 6 digits, and then went to 7 at some point.

Way before my time, but I think I read it somewhere a long time ago.

I lived in Wyoming in the early 90's, in a small town, and we could dial other folks in the town by only dialing the last 4 digits, as the prefixes were all the same. That stopped in mid 90's, and we then had to dial 1, plus area code and the entire 7 digits, to get even the next door neighbor.

Anybody remember this stuff?