C&H Precision BUIS (Back Up Iron Sights) were specifically designed to be used with the OEM Optic Ready Slides and Direct Milled Slides. LowWitness is defined as a BUIS (Back Up Iron Sights) that sits between .075 to .100 into the optic FOV (Field Of View) which is equivalent to lower 1/6 to lower 1/8. True Co-Witness blocks half of your red dot visible window / FOV while Lower Third blocks one third of your visible window / FOV. Red Dot Industry Experts believe Back Up Irons should not be a vision blocker and should be as low as possible but still useable if needed. LowWitnessA(R) Back Up Iron Sight heights provide you the lowest usable sight height without being a vision blocker "Everything you need, nothing you don't"