Bulletproof glass (ballistic glass, transparent armor, and bullet-resistant glass) is a strong and optically transparent material that is particularly resistant to penetration by projectiles. Like any other material, it isn’t completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so it can flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from 3⁄4 to 3 1⁄2 inches (19 to 89 mm). Bulletproof glass is used in windows of buildings that require such security, such as jewelry stores and embassies, and of military and private vehicles.