If your head whipped around on Interstate 5 near DuPont on Thursday and you thought, “What was that?” when you passed a sign reading “Joint Base Lewis-McChord (JBLM) next 6 exits,” it wasn’t your traffic-befuddled brain.

It was a brand new highway sign, heralding change at the South Sound’s two big military bases. Fort Lewis and McChord Air Force Base are merging into one installation Monday.

The first sign of the change went up on northbound I-5 at Milepost 118, state Department of Transportation spokeswoman Kelly Stowe said.

It will be followed Monday with an identical sign on southbound I-5, about 500 feet north of the Highway 512 interchange near what is now the main entrance to McChord Air Force Base – which will henceforth be known as JBLM McChord Field.

The two freeway signs cost about $10,000 in labor and materials, with the cost to be shared by the Department of Transportation and the Department of Defense, Stowe said.

The Army post and the Air Force base are merging operations as the result of a 2005 order from the federal Base Realignment and Closure Commission. They’ll be one of 12 such joint military installations across the nation.

Merging support operations does not alter the mission of either military branch, nor change its basic chain-of-command structure. There will, however, be a new garrison to manage the operations.

The milestone will be marked on Monday with a 10 a.m. ceremony casing the colors of the Army garrison that now runs Fort Lewis and unfurling the new colors of Joint Base Lewis-McChord.