12 generally flat to permit ready passage of a plurality of pens over one another having an elongated stylus tube 14 extending in the lengthwise direction from body 12 and terminating with a writing nib 16 at a bent front portion 18 of stylus tube 14 the underside of body 12 includes as shown in phantom in fig1 a lengthwise slot 20 provided by channel members 20a and 20b adapted to slidingly receive a pen arm along the length of the underside of body 12 body 12 further including no other interfering configurations to preclude the sliding engagement of a mating pen arm along the whole length of body 12 also seen in fig1 is pen arm 22 comprising in accordance with the present invention an elongated blade 24 with mounting means 26 at the rear end of blade 24 for mounting pen arm 22 in an instrument and a positioning means namely a slot 28 at the forward end of blade 24 for positioning the forward end of blade 24 at the nib end of the mating stylus tube intermediate the ends of stylus tube 14 in the plane formed by bent portion 18 and stylus tube 14 bends 30 and 32 are provided to facilitate inter pen passage of a plurality of such pens used in combination blade 24 includes corresponding bends 34 and 36 so that when marker 10 and arm 22 are assembled as shown by the arrow in fig1 the stylus tube 14 of marker 10 traverses essentially the same lengthwise path as blade 24 of pen arm 22 in this manner the vertical space required by the stylus tube pen arm.