Abstract: The cap for the writing instrument comprises a jacket mounted slidably on a sealing insert or sleeve which provides a sealed moisture chamber for the writing tip when the cap is on the writing instrument. Near the writing tip there is a circular bulge on the shaft of the writing instrument which enters into a sealing engagement with an interior passage of the sealing insert. On putting the cap on the writing instrument the sealing insert is pushed over the circular bulge until it contacts a contacting stop on the jacket. Then the circular bulge presses tightly into the sealing insert and it locks the jacket to the writing instrument so that the entire arrangement is kept in that position until the cap is removed. A signal slider is attached with the sealing insert which is extendable out from the end of the jacket in order to signal in its extended position the proper seating of the cap on the writing instrument. Erroneous indications of a sealed state are prevented with our invention.