Reading the pattern
To read a pattern, measure from the center of one screw hole to the center of the next, across and then down. 100 × 100 means the holes form a square 100 mm on a side. 75 × 75 is 75 mm. A ruler across the back of the screen is all you need.
The whole family is called the Flat Display Mounting Interface, or FDMI, and it's also known as the VESA Mounting Interface Standard. The first standard came out in 1997, and it's now known as part D. Part D covers the 75 and 100 mm patterns, and both use M4 screws.
What each size can carry
Bigger patterns carry heavier screens. These are the maximum weights the standard lists, with the smallest screen each size is meant for:
- 75 × 75 mm, part D: up to 17.5 lb (8 kg), screens from 12 inches (30 cm)
- 100 × 100 mm, part D: up to 30.8 lb (14 kg), screens from 12 inches (30 cm)
- 100 × 200 mm, part E: up to 50 lb (22.7 kg), screens from 23 inches (58 cm)
- 200 × 200 mm and up, part F: up to 110 lb (50 kg) with M6 screws or 250 lb (113.6 kg) with M8, screens from 31 inches (79 cm)
Smaller parts, B and C, cover screens as small as 4 inches (10 cm).
Those are limits for the pattern, not for the arm you bolt to it. An arm has its own weight range, and you need to stay inside both. The screw lengths in the standard's table leave out the bracket, so a thick plate or spacer means longer screws.
What a mount does for your desk
A monitor with a VESA pattern can leave its stand and go on an arm or a wall bracket. That frees the desk surface under the screen and lets you set the height and distance to suit you. An arm doesn't make the screens any narrower, though. Two 27-inch panels side by side add up to 47.1 inches (119.5 cm) of width, before you count bezels.
The Desk Setup Visualizer lays monitors out on a desk of your size, so you can check the width before you buy arms.