The single silver bar used as the United States Air Force First Lieutenant Rank Insignia was inherited from the U.S. Army system of officer grade insignia when the Air Force was created as a distinct branch of the U.S. military under the Department of Defense in 1947. Nonetheless, the same question is invariably asked when a new new generation of officers reaches the O-2 rank of 1st Lt: Why is the lower-ranked 2d Lt insignia made out of gold and the 1st Lt insignia made out of silver, when gold is almost universally assumed to be more precious than silver?
The answer is a convoluted one, involving shoulder straps, epaulets, borders, Majors and Lieutenant Colonels, and the two dominance of the infantry in the early days of our country’s fighting forces. But the interesting point in the saga actually came during World War I, when the Army had no insignia for the waves of 2nd Lieutenants pouring into the ranks after America entered the conflict. At that time, 1st Lieutenants had the single silver bar and Captains had two attached bars, and a seemingly simple suggestion was made: why not add a bar to each of their insignias and give the “lowly” 2nd Lieutenants a single, silver bar?
More 1st Lieutenant Insignias and DevicesAs reasonable as that might sound, there was a lot more work involved in changing the insignias for two existing ranks rather than coming up with a single new one. So, borrowing from the same concept that led to the lower-ranking Majors having a gold oak leaf versus the silver oak leaf of the higher-ranking Lieutenant Colonel, the Army settled on a single gold bar for its lowest-ranking Commissioned Officers—and the Air Force simply adopted the system when it was birth some four decades later.
And yes, it's called a "butter bar" regardless of the branch of service...