Abstract: A blade is carried in a housing which is adapted to slide along a guide rail with the blade depending from the housing and making sliding contact over at least a portion of the length of the blade with a blade guide which runs parallel to the guide rail. A portion of the blade protrudes below the blade guide to engage in and cut a cardboard mount or the like situated below the guide. The blade is inclined so that while the cutting edge is parallel to and slides along the blade guide the plane of the blade is itself inclined to the said blade guide surface. The blade is detachably secured in the housing for replacement and is arranged so that one corner of the cutting edge protrudes below the level of the blade guide. The blade mounting is adjustable within the housing so that the amount by which the corner of the blade extends below the blade guide can be adjusted to control the depth of cut.