The present invention provides a clamp (1) for holding a workpiece (12) that is particularly suitable for use in CNC machining. The clamp (1) is particularly useful for use in picture-frame machining techniques as it allows a workpiece (12) to be held in a position wherein a machine can have substantially unencumbered access to both sides of the workpiece (12) without the need to remove the workpiece (12) from the clamp (1). The clamp (1) comprises a frame comprising a base (2), a first arm (3), and a second arm (4), the first arm (3) and the second arm (4) each mounted to the base (2) at an inner end such that they are parallel with one another. The clamp (1) further comprises a set of opposed vices (5, 6, 7, 8) mounted at facing sides of the frame to hold a workpiece (12) therebetween, the set of vices (5, 6, 7, 8) consisting of exactly three self-centering vices (5, 6), two mounted at the first side of the frame and one mounted at a second opposing side of the frame, and optionally one or more floating vices (7, 8). In embodiments of the invention the first arm (3) and the second arm (4) are mounted to the base (2) at an inner end by a vice mechanism such that they are parallel with one another and are movable towards and apart from one another. The present invention also provides a method of using the clamp (1) for machining.