Explanation of Co-Witness

[SinnFéinM1911]

Co-witness = seeing two at the same time, regardless of plane.. equal to or bottom 1/3. The key point is being able to rapidly shift to irons in the unlikely event your optic fails.

An EoTech on a riser will put the irons below the red dot, many Aimpoint mounts do this by design. You then have to zero Irons & Red Dots seperately.

Many folks get lazy and just zero the dot & move the Irons to it.... or vice versa. The validity of this zero depends on the plane of the optic/dot and the irons. Best to zero one then the other seperately.

I have also heard as it as:

A ability to quickly sight in your Iron after your Optics / Red Dots BZO'd, and adjsut your iron sights or buis to that point (seems kinda lazy to me).