The four holes are a shared standard, not a brand feature
The mounting pattern on the back of a monitor is not something each maker invents. It is a published standard, VESA's Flat Display Mounting Interface, so an arm built by one company and a screen built by another are drawn to the same measurements and simply meet. That is why you can bolt a no-name arm to a Dell or a Samsung without either one knowing about the other.
For a desktop monitor the pattern is a square of four threaded holes, and it comes in two sizes: 75 mm to a side or 100 mm to a side. The standard scales up from there, into wider multi-hole patterns meant for larger displays and televisions, but the desk end of it is those two small squares. VESA sorts displays by size and weight, and an ordinary desktop monitor, weighing up to about 14 kg, sits in the band that uses the small four-hole square. So the real question behind "is my monitor VESA compatible" is "does it sit in that mainstream band", and almost every screen bought for a desk does.
Finding the pattern on the back of your screen
Plenty of monitors hide the holes rather than advertise them. The factory stand often clips straight over them, a small plastic panel can snap across them, or they sit in a shallow recess you have to look into. Take the stand off first, which is usually one release button or a couple of screws, and the square of holes is normally right behind where it sat.
Telling 75 from 100 takes a tape measure, not a spec sheet. Measure centre to centre between two holes that sit side by side: a gap of about 7.5 cm is the smaller square, about 10 cm is the larger. You do not need to be exact about it, because an arm sold for desktop monitors is drilled for both squares in the same plate, so either measurement lands on a hole it already has. If the back is smooth with nothing to bolt to anywhere, that is the no-holes case further down.
The second number: what the arm can hold
The pattern only decides whether the arm can attach. Whether it then stays put is the second number, and it is the one buyers skim. Every arm is rated for a weight range, and your panel has to sit inside it. Take the weight from the monitor's spec sheet, and use the figure without the stand, since the stand is coming off. Then give the rating room to spare. A panel hung near the top of what an arm will hold works its joints to the limit, so the head tends to creep downward as the day goes on, where the same panel on a stronger arm holds its place.
There is a lower limit as well as an upper one. Sturdier gas-spring arms set a floor as well as a ceiling, rated from a few kilograms upward, so a very light screen can fall below the minimum and the spring never settles, floating the monitor back up instead of holding it. Width matters too. A wide or curved panel swings a longer lever off the mount than a compact screen of the same weight, so an ultrawide wants an arm that names its size in the rating, not one it merely scrapes onto on paper. Once the pattern and the weight both check out, our best monitor arm guide picks three arms across that envelope, from an everyday screen to a heavy ultrawide.
When a monitor has no holes at all
A few screens genuinely have nothing to bolt to. Slim all-in-one style monitors are the usual culprits, along with a scattering of budget panels built down to a price, and of course a laptop, which was never meant to hang off an arm in the first place. For a monitor, the way round it is an adapter: a bracket, or a pair of adjustable jaws, that grips the screen by its edges or clamps behind it and then presents a standard hole pattern for the arm to bolt onto.
Two honest cautions come with that route. The adapter is its own purchase, not a part the arm brings in the box, so budget for it separately. And gripping a screen from outside adds a little depth and a little wobble that a direct bolt-through does not, so a monitor that already has holes is always the tidier mount. If you are working on a laptop rather than a desktop screen, you can skip the mounting question entirely: a laptop stand lifts it towards eye level without any bracket at all.
Clamp or grommet: the desk has to fit too
Fit is not only about the monitor. An arm has to grab the desk, and it does that one of two ways, so the desk gets a vote as well. A clamp pinches the desk edge between a top plate and a screw pad from below, which suits most desks that have a free, solid edge a few centimetres deep. A grommet mount instead drops a bolt through a hole in the desktop and tightens underneath, which is the answer when the edge will not take a clamp, because it is pushed against a wall, thicker than the clamp opens, or a sheet of glass a clamp would strain.
Better arms list both fittings and include the parts for each; a cheaper one may name only the clamp. It is worth checking which one your desk needs before you order, because the wrong fitting means the arm turns up and will not go on at all. Our guide to whether an arm fits your desk measures the edge out in full and covers glass tops and the grommet route. If lifting the screen off its stand leaves a tangle of cable on show, which it usually does, the tidy-up is its own small job that our desk mat and cable management guide covers.
Once it fits, put it at the right height
Confirming the arm will fit is the boring half. The point of all of it is the freedom that follows: with the screen off its factory stand you can finally set the height to your eyes instead of living with wherever the stand left it. Where to set it is a separate question with a clear answer of its own, and how high your monitor should be is the piece that settles it. Getting the fit right first is simply what earns you the room to.
FAQ
Q: Do I need the 75 mm or the 100 mm pattern? A: You do not choose it, your monitor already has one or the other. It rarely matters in practice, because almost every arm sold for desktop screens is drilled for both squares in the same plate, so an arm listed as "VESA 75/100" bolts to either. Measure the gap between two neighbouring holes to know which you have: roughly 7.5 cm is the smaller square, 10 cm the larger.
Q: Can I add VESA holes to a monitor that has none? A: Not to the screen itself, but you can bracket around it. An adapter plate or a set of clamping jaws grips the monitor and gives the arm a standard hole pattern to bolt to. It works, but it is an extra buy and adds a little depth, so a screen that came with holes is always the cleaner mount.
Q: Does an ultrawide or curved monitor use a different pattern? A: Usually the same 100 mm square, so the holes rarely stop you on an ultrawide. The size does the deciding instead: a broad panel loads the mount far more than its bare weight implies, which is why a curved 34-inch panel belongs on an arm rated to that width, not one it only just clears.
Q: What screws hold the monitor to the arm? A: The 75 and 100 mm patterns take an M4 machine screw, and a decent arm ships with a set, often in a couple of lengths, because how far the screw has to reach depends on how deep the holes are recessed. If you ever need them loose, M4 is the thread to ask for, but check the arm's own box first, since it normally has them.