Co witnessing refers to using iron sights along with your holographic sights. You will come across two types of co witnessing, Absolute co witness, and lower one-third co witness. Absolute co witnessing doesn't require you to adjust your head too much as the holographic reticle is in front of the iron sight. However, in the lower one-third co witness, the front sight post shows up in the lower third of the glass. The reticle, however, remains in the center of the holographic glass.