The gauge system predates using inches or millimeters as standard measurements and dates back to a British way of measuring wire thickness. It’s not clear when or why it was adopted for sheet metal, but it remains the standard today. Higher numbers are thinner. So 14 gauge is thicker than 18 gauge. Confusingly, it also varies from one metal to another: 18-gauge steel is .0478 inches, 18-gauge stainless steel is .050 inches, and 18-gauge aluminum is .4030 inches. Minor differences for sure, but important for some users.