A writing instrument comprises a substantially tubular body extending between a top and a writing end, and an elongate slot disposed in an outer surface of the body and having a detents defined at each end of the slot, and a tubular open-ended cap disposed coaxially around the body and having a cut defining an elongate arm connected to the cap at one end and having an inwardly-oriented protrusion at another end and sized to engage the detents of the slot. The cap is slidable along the body such that the inwardly-oriented protrusion slides along the slot until it engages a detent at an end of the slot thereby arresting the cap either in a closed position, in which the cap covers the writing end, or in an open position in which the writing end is exposed.