Question
We have a beautiful stone wall about 4' high behind our house.[approx. 10' back from our house. Our neighbor is appox. 10' back from the wall on the other side!] We were looking to plant aborvitaes along part of the wall for privacy. I saw somewhere in my travels a similar wall with trees planted in front and all the bottom branches were trimmed off so you could see the wall. the Trees actually looked like they were "sitting" ontop of the wall. My question is, when would I trim off those bottom branches to expose the wall?  

Answer
Nora,
I would not recommend doing this kind of pruning until the Arborvitae are at least two to three times the size of the wall (8-12 ft.). This way you are not removing more leaf tissue than the plants can handle. Pruning back too severly can stress the plants, causing them to become weak and more insect prone.
I wish you luck.
Ed Gulliksen