Abstract: A clip-on marker for electrical or optical leads or cables such as petrol engine spark plug leads having an elastomeric outer sheath comprises first and second half-shells that hinge together along one edge and clip together along the other edge to tightly encircle the lead. One half-shell has on its external surface raised or indented regions that are readily perceptible by touch and whose number identifies the marker. The other half-shell may carry a legible identifier. Markers may be moulded in sets, e.g. of six, via a single mold sprue.