Several folks have been looking for a way to have rotatable monitors for vertical and horizontal games.  Why?  Because the monitor is a rectangle, and not a square, if you try to run a vertical game in a horizontal monitor, you don't get full screen.  Similar problems occur with running a horizontal game in a vertical orientation.

There are two main considerations with rotating monitors.  The first, of course, is how to physically do it.  The second is consequences of rotating - specifically distortion of the screen when rotating.

I haven't done this myself (... yet!), so this page is a collection of information I've found elsewhere.

How to?
Problems & Resolutions

This section consists mainly of pointing you to examples done by folks elsewhere.

Mark Jenison's Mark 13

The Mark 13 is a Jamma based arcade cabinet constructed by Mark Jenison and I believe Rick Schieve.  It sports swappable control panels and a rotatable monitor mount.  Mark runs M.A.R.S. - Mark's Arcade Retro Site and it's a must visit on the arcade enthusiast's tour of the Internet.

Mark's write-up of the rotating mount for our FAQ

pictures on Spies
  (A few of the close-ups of the monitor are missing, I'm trying to track them down)

Mark's page

Mike's MAME cabinet has a very nice write-up of how he mounted his monitor so it could rotate.  The pictures are very nice as well.

Mr. Salty's MAME cabinet is one of the more inspirational cabinet's I've seen, with a well documented section on rotatable monitors.  It is also one of the coolest web sites I've seen in a long time :)

Rocketcade is an often updated work in progress featuring a MAME cabinet with a rotatable monitor.  The links don't all work yet, but the what's new page documents the construction progress.  They've experienced some of the problems we'll discuss below, and have (Aug. 30) just recently ordered a degaussing coil.  Looking forward to hearing about their results.

The LillyPad is a slick arcade cabinet with a rotating monitor inspired by Mr. Salty's cabinet.  Also a very slick website :)

Here's another one, this time from someone who says they wouldn't hvae a rotating monitor if they had it to do all over again -- nontheless they have a good writeup of how they did it.

Got a rotating monitor project not listed here?  Please let me know so I can include it!

There appear to be two main hurdles to overcome aside from the physical mounting.  The first is the problem with heat dissipation with monitors oriented differently from upright (in fact, heat dissipation should be a consideration for your entire project), and the second is color distortion from rotating.  The bulk of this section comes from mailing list discussions with folks who know quite a bit about the arcade industry.  Because I have not specifically obtained permission to include their names, I have not done so.  Should I obtain their permission I'll include proper credit.

Heat dissipation

If you are using an enclosed computer monitor in your project, one thing to consider is heat dissipation.  Some monitors don't have adequate heating vents on the sides which make the monitor run hotter.  Combine that with the fact that you're stuffing them in a cabinet, and you may run into heat problems.  Of course if you aren't using a monitor surround then you're OK regardless.  Make note of how your monitor is ventilated, and add extra ventilation, fans, or remove the housing as seems best.

Monitors, unless specifically designed to rotate, are meant for one orientation.  They are calibrated so that the magnetic field and horizontal/vertical orientation they are in will not distort the colors.  If you change the orientation, or move the monitor somewhere that the magnetic field is significantly different, the colors will likely distort.  This is not normally a permanent condition, and can be fixed by degaussing.  There are two good links to degaussing information at the end of this section - however, degaussing is essentially the process of demagnetizing the monitor so that the electron rays strike the proper color phosphor, preventing color distortion.

Degaussing can be done in a couple of ways.  First, most monitors include a degaussing coil built in.  When you first turn the monitor on, the degaussing function activates for a few seconds.  After it's been on, the monitor must cool down for approximately 15 minutes or so before the degaussing will function again.  The second way is to use an external degausser.  This is a device you gradually bring closer to your monitor, then back again, while it emits a magnetic field that degausses the monitor.  See the following links for in depth discussions on degaussing.

RandyFromm technical department - How degaussing coils work
Sci.Electronics.Repair FAQ - Degaussing a CRT

Here are some comments from the folks on the mail list discussion:

Comment 1

If the game has been off for a good amount of time (maybe 20+ minutes), I can rotate the monitor in any direction, and the colors will be fine.

However, if I go from playing a Horizontal game to a vertical game, as I turn the monitor, the colors will change.  By using a degausing coil, I can fix 90% of the monitors color in about 15 seconds, but for some reason the upper right corner (which would be the lower right corner in horizontal mode) remains unchanged :-\.  Of course, if I turn the game off and turn it on a few hours later, the corner color is fine again.

So basically, you can rotate the monitor as frequently as you like if you use a degaussing coil, but even if you do, you may still have some minor color oddities on the outer edges.  Minor, IHMO, but YMMV.

Comment 2

To add to this discussion further...

In a house basement the effects you describe are much worse.  My games don't like being put to face a certain direction in my basement and no amount of degaussing helps at all.  I had the same problem in the last house I owned so I assume this has something to do with magnetic fields generated by the urrounding
soil/rock.

Comment 3

There is a really easy way around the problem of having to wait for a monitor to "straighten itself out" by letting it sit for a while..  The reason you have to wait is because the PTC is cooling down. Just short it out and put a switch inline with the degaussing coil (DGC). Whenever you want to DG just press the button for about 3 seconds. DGing doesn't hurt the monitor, just dont hold the button down for a few minutes or you may see smoke.

The PTC (positive Temperature coefficient) is basically a resistor that limits current flow when it heats up. When the monitor is cold the PTC passes current to the DGC, and after a few seconds it heats up and cuts the current off. It will "trickle" enough to keep it warm (and thus the  DGC off) whenever the monitor is on. it usually takes at least 10 min for the PTC to cool off enought to do a complete DG. The PTC is that little black box with 3 pins on the monitor chassis, usually right next to the DGC connector..

saint's note - if this didn't make sense to you, you should not try it!