Co-witnessing means setting your iron sights at a height where you can see and use them through a red dot window. It gives you a fallback if the optic fails and, just as usefully, a built-in reference that tells you at a glance whether the dot is where it should be. Two arrangements are common, and mount height is what decides which one you get.
Absolute co-witness
With an absolute co-witness, the dot sits directly on top of the front sight post when the irons are aligned. Look through the optic, bring the irons into alignment, and the dot is sitting right on the tip of the front post.
On an AR-pattern rifle this corresponds to a mount height of about 1.4 inches from the rail to the center of the optic — the "lower" of the two standard heights, and the one most red dots ship with.
The advantage is confirmation. If your dot ever looks like it might have shifted, you can verify it against the irons in a second without firing a round. It is also the more intuitive arrangement for shooters coming from iron sights. The cost is that the front sight post sits in the middle of your window, and on a small optic it occupies a meaningful share of what you can see.
Lower one-third co-witness
A lower one-third co-witness uses a taller mount — roughly 1.93 inches on an AR — so the irons appear in the bottom third of the window with clear glass above them.
For normal shooting the dot floats in an open, unobstructed field of view, and the irons are still there when you drop your head slightly to find them. Most shooters who run both arrangements end up preferring this one, and it is the dominant configuration on modern carbines.
The taller mount brings a second benefit that has nothing to do with the irons: it raises your head into a more upright, neutral posture behind the rifle, which is more comfortable in body armor, easier to sustain, and generally faster to acquire. This is why the 1.93-inch height has spread even to rifles with no iron sights at all.
What you actually need to buy
Two things have to match: the sights and the mount.
You need suppressor-height, also called optic-height, iron sights. Standard-height AR sights sit too low to appear in a red dot window on any modern mount. Folding versions keep the window clear until you need them and are the usual choice.
Then you need a mount of the correct height. Quality mounts state which co-witness they produce — most absolute mounts are around 1.4 inches, most lower one-third mounts around 1.93 inches, though the exact figure depends on the optic's own base height. Buy a mount that names the co-witness rather than guessing from a number.
Sight-over-bore, and why your close shots hit low
A taller mount increases the distance between your line of sight and the bore axis — on a 1.93-inch mount, well over two inches. The bullet starts that far below your line of sight and has to climb to meet it.
The practical result is that at 7 to 10 yards you will hit roughly two inches low if you hold dead center. That is a hit on a torso and a miss on a small target, and it is the reason close-range precision drills reward knowing your offset rather than discovering it. Shoot a target at 7 yards and measure it once; it will not change.
Setting it up
Zero the dot first and treat the irons as a separate, secondary zero — do not try to adjust the dot to match the irons, which just moves your real zero around. Many shooters zero the dot at 50 yards, which gives a useful near-and-far crossing on a 5.56 carbine, then zero the irons independently and leave them folded.
The zeroing principle is the same one covered in how to zero a red dot: shoot a group, move the dot to the center of the group, confirm with a fresh group.
Do you need irons at all?
It is a fair question. Modern red dots are extremely reliable and run for years on a battery, and plenty of serious shooters run a bare optic to save weight and keep the window clean. Others carry a small offset dot mounted at 45 degrees instead of irons, which is faster to transition to and works at any light level.
The honest answer is that irons cost a few ounces and solve a rare problem, and a lower one-third setup costs you almost nothing in sight picture to have them. For a defensive rifle, the redundancy is cheap. For a competition or hunting rifle, it may not be worth the weight.
On a pistol
The same logic applies with different hardware. Suppressor-height sights on a slide-mounted optic give you the identical fallback, and tall front sights also make it far easier to find the dot during the draw while you are still learning the presentation. Read pistol red dots and optic footprints before buying anything, since the slide cut determines what fits.
Browse rifle dot sights and mounts to build the setup.