Walls and Ceilings - Sheetrock orientation

Is there a particular reason the installation instructions I see show sheets of wallboard being installed horizontally rather than vertically? Or does it matter?

I am not a "rocker". Imagine a 12-foot long wall 8 feet tall. Two sheets of 12-foot drywall will cover horizontally with one 12-foot seam between them to mud and finish. Installing three 8-foot sheets vertically would require 16 feet of seams to be mudded and finished. Also fewer screws needed for a 12 foot seam versus a 16-foot seam.

However, vertical installation would be easier physically due to handling less weight per sheet. Also, there would not be a beveled edge at the bottom, makes installing baseboard more challenging. I am sure the pros will have other reasons.

It's easier to jamb the sheetrock to the ceiling rock and get a good line and you eliminate finish lines when you lay it horizontally. Your second sheet will be just a little above the floor and that's fine to prevent the possibility of wicking. If installed correctly, your light switches will fall across the joint and will be easier to locate and cut out.
To your question.....it don't matter. It is the way the pros do it, because they have 14 people doing it at one time and it's no problem lifting the 12' sheets into place, and it cuts down on their finish time.

And don't forget the actual taping..is it easier to work horizontally at the 4' mark or vertically from the ceiling to the floor?

Besides being easier to finish when hung horizontally, it's easier to hide any discrepancies in the framing when hung horizontal. Can make a difference on a long wall.

Thanks! That about covers it . . .