If you've shopped red dot mounts you've seen terms like “absolute co-witness”, “lower-1/3” and “1.93in”. They describe the height your optic sits above the bore — and it genuinely affects how your rifle feels to shoot. Here's a clear explanation.
What is co-witness?
Co-witness means your iron sights and red dot can be seen and used together through the optic window — so if the dot fails, your irons are right there as backup. The mount height determines where the dot sits relative to the irons.
The main heights
- Absolute co-witness: the dot lines up exactly with your irons in the centre of the window. Maximum redundancy, but the irons sit in your sight picture.
- Lower-1/3 co-witness: the dot sits slightly higher, so irons appear in the lower third of the window. The most popular choice — a clean dot picture with irons available if needed.
- 1.93in (and 1.7in) height: taller mounts that raise the optic for a more heads-up, comfortable posture (popular for speed and reducing neck strain). Irons typically won't co-witness at these heights.
Which height should you choose?
- Backup irons are a priority: absolute or lower-1/3 co-witness.
- Cleanest dot picture with irons available: lower-1/3 (the all-round favourite for AR-15s).
- Most comfortable, heads-up posture: a 1.93in mount — increasingly popular even without co-witness.
Pistol optics are different
On pistols, height is set by the slide cut and any adapter plate, and suppressor-height irons are used to co-witness. The rifle heights above apply to AR-pattern and similar rifles.
Getting it right
Pick a quality red dot mount in your chosen height, and if you're pairing a magnifier, match its height to the optic. Compare the exact optical heights side by side in our red dot mount height chart. Then mount and zero correctly — see our mounting and zeroing guide.
Browse red dot sight mounts, red dot sights and the full AR-15 optics range.
Frequently asked questions
What does lower-1/3 co-witness mean?
The optic is mounted so your iron sights appear in the lower third of the window — giving a clean dot picture with irons available as backup. It's the most popular AR-15 height.
Absolute or lower-1/3 co-witness?
Absolute aligns the dot exactly with the irons; lower-1/3 raises the dot slightly for a cleaner picture. Lower-1/3 is the more popular all-rounder.
What is a 1.93in mount?
A taller mount that raises the optic for a more comfortable, heads-up shooting posture — popular for speed, though irons usually won't co-witness at that height.