The lowdown on the gadget affixed to their heads.

Several Slate readers have inquired about the black gizmos affixed to the helmets of U.S. soldiers. What's the purpose of these curious protrusions, which look a bit like the cobra atop Tutankhamen's mask?

They are night-vision goggle mounts. As the name implies, they are designed to hold in place the binocularlike goggles that bathe the night in a bright-green glow—from the user's point of view, that is. That way, soldiers have both hands free to handle their weapons.

NVG mounts have two positions: up and down. The up position is the stowed, or daytime, position, which gives the wearer that Pharaonic look. When it's time to sally forth after dark, the mount can be rotated downward with a simple tug. The Army's latest version attaches to the standard-issue Kevlar helmets by means of a retention strap and metal clips. The clips hook onto the helmet's back and front. (Click here for a look at an NVG mount in the full-ready position.)

Some older NVG mounts, by contrast, actually screwed into the front of the helmet, making them difficult to fix or replace in midcombat. A strap-and-clip system has become more feasible as the weight of NVGs has decreased; the standard Army NVG unit now weighs just 1.5 pounds.

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Explainer thanks Mark Reynosa, author of U.S. Combat Helmets of the 20^th Century.