Kilroy’s job was as an inspector; one of the aspects of the process he checked were all the rivets that were involved in holding the ship together. They had to be put in properly and fastened solidly. As Kilroy made his inspections—often going into tight spaces and down into tanks—he counted the blocks of rivets as he went. He then used a waxy chalk to leave a checkmark as to the area that he had approved.
Riveters were paid on a piecework basis, with their pay calculated by the rivet. After Kilroy left for the day, the workers sometimes erased the mark so that the inspector on the next shift would come through and count their work for a second time. This would increase their pay.
After a time, one of the shipyard supervisors called Kilroy in to discuss the circumstances. The count of ship parts completed seemed below what it should be, considering the number of rivets inspected.
Kilroy thought through the circumstances. He realized someone must have been tampering with his checkmarks. He considered the options. Using paint to make his mark would be tamper-proof, but it would be difficult to get in and out of some of the spots that needed to be inspected if he were carrying a paint can. He decided to try to maintain his system with an addition: He left his checkmark but began to leave “Kilroy was here” in over-sized letters to make the tampering more difficult. Later he added the sketch of the fellow peering over the fence.
This addition—and perhaps word-of-mouth around the Quincy shipyard—got a message through to the riveters: don’t tamper with the inspection count.
Normally all inspection marks would have been covered when the ship was painted before launch. But because of the urgency of the war, ships began leaving the coast with “Kilroy was here” marked in various locations of the ship.
Servicemen everywhere began seeing the signature and drawing but they hadn’t a clue as to the meaning behind it.