On Fri, 18 Aug 2017 11:44:02 GMT, Iggy
m wrote:

replying to clare, Iggy wrote:
That's what I initially thought, until I tried it and broke it down. A
Vertical's seam is 100% backed and the full panel's entire perimeter edges are
as well. However, a Horizontal seam only has a 1-1/2" support every 14-1/2"s.
That's only about 10% of a seam that's backed. And, no-one does nor would
waste the time to properly put any back-blocking behind the seam, which should
be required.

I can kick and hammer a Vertical seam and nothing happens. But, with
Horizontal I can just moderately lean against it to deflect it open and
completely crack the seam open and loose. I've heard your point of better
seam-hiding before and frankly I've only rarely witnessed it. I use just a 10"
knife on my seams and then lightly sand with a block or pole sander's plate.
No gaps, totally flat and entirely invisible, always.

You might enjoy My List. It's here, at the end/bottom:
https://www.homeownershub.com/mainte...ll-318143-.htm

If the first layer of joint compund is a "setting" compound, with the
"drying" compound just used to finish, the joint won't separate when
you lean on it. Dutabond 45 or durabobd 90 is made for that
application. Use it carefully because it is hellishly hard to sand -
it cures like concrete or plaster of paris, rather than drying like
"drywall mud". It is also 100% fire resistant - better than the
drywall itself.
This is what I meant by "properly installed"

If you want full fire rated, install 1 layer vetically with screws,
not mudded - then install the second layer horizontal , preferably
with adhesive, and mud.

Commonly done on "shared" walls in wood-framed multi-unit residential
buildings as a "fire break"