Generally I mount square prints almost identically to DWThomas, same image & frame size except with no white border around the prints, matting to the edge of the image.
How much I offset from top & bottom (which I think is your question Ralph) depends on how the image is going to be displayed. My exhibition work is a mixture of negative formats 5x4(10x8), 6x6 and 6x17 (all full frame) in the same 20x16 frames so I spent some time doing visual comparisons of image size and in frame positioning with each format, first on the PC like Ralph's drawing, then made actual prints. It works out that the vertical centre of each image size is constant for landscape format, because that seems to work best visually.
More recently I've been selling some square images in square frames, initially because a local store (UK) has some very nice square frames at bargain prices, but they great when finished and hung on a wall.
I prefer the square image set towards the bottom, leaving a larger, white area above, for more "air", no matter what the subject.
To me doing it the opposite way looks weird.
As mentioned, ready-made square frames are not very common but whole frame supply companies for do-it-yourself framers like my company have no problem providing square frames or heavily bottom weighted mats such as in the examples above. In general, larger mat borders tend to give the image more of a gallery feel. For a square image 11x11 image, equal 4.5 inch mat borders resulting in a 20x20 inch frame would have nice gallery feel. Off the shelf frame sides are commonly available in 20 inch segments although still need a custom mat.
The heavy bottom weighted mat, where the bottom border is significantly larger than the top and side borders will also offer a nice gallery flare and in fact Nielsen actually makes a couple ready-made frames for 8x10 images in 16x20 frames. You can see pics here, and I have included the mat border dimensions at the bottom of the text on these two pages:
Nielsen Gallery Collection 8x10 (16x20) GF1950E
Nielsen Gallery Collection 8x10 (16x20) GF1950D
Nielsen use to carry an 11x14 frame with 5x5 opening but they discontinued it. Bottom weighted mats are almost never offered in ready-made frames because as you notice with the 8x10 you have to manufacture and stock two different sizes.
If you are going to bottom weight the top and side borders should be equal, or the top border should be within an inch of the side borders. If the frame is 11x14 or smaller I would suggest the top border be less than 1 inch larger than the side borders if not equal.
I think that might work well for certain types of images. Just as in photography, there are no official framing rules, just guidelines.
Originally Posted by Erik Prestmo
If you're not already familiar with this site, take a look. http://www.russellcottrell.com/photo/centering.htm
Originally Posted by RalphLambrecht
My mounts are similar to Dave Thomas's, but I tweak them a little from Cottrell's dimensions.
Steve, looking at the link John's just posted that's quite a neat guide that pretty much mirrors what we do visually.
Originally Posted by Sirius Glass
Thanks for the compliments. I was targeting "optical centering" which raises the image a little bit above the centerline, purportedly adding weight to the bottom (so it doesn't get hung upside down!) It can be done graphically, or I have used a handy html program from this guy (he shows the graphical method too). In all honesty, I am beginning to think I might lean toward positioning just a tad lower, some of that may come from whether or not one adjusts the calculated value for the overlap of the frame.
I should add, I have seen images that were almost panoramic horizontals mounted in a rectangular frame held vertical and with the bottom of the image almost on the centerline. There seems to be some occasional benefit in breaking conventional rules to achieve an effect. Somewhere years ago I heard of a competition that specified all work was to be in 16x20 frames too!
I occasionally frame square, but since I don't sell much work, I've lately made 16x20 a sort of standard that works for most of my photos and on the rare occasion, a quarter-sheet watercolor so I can recycle frames.
Edit: Oops! Looks like I got antsy and jumped in without reading to the end, sorry for the redundant info.
I like the idea of keeping top and side margins the same width as shown in Mark's second link in post #13.
Originally Posted by jovo
Thanks you. I'm very familiar with it. It's an old but proven technique that works well in most cases.