referring to the drawing , a preferred dial caliper , generally indicated at 10 , comprises an elongated metal beam 12 having a channel 14 . an elongated rack 16 is disposed in the channel . the beam carries a jaw 18 which extends from one side of the beam , and a second longer jaw 20 which extends from the opposite side of the beam . jaw 18 has a linear measuring edge 22 on the outside of the jaw , whereas jaw 20 has a linear measuring edge 24 on the inside of the jaw . slider frame 26 is mounted on the beam and has a rigidly afixed depth rod 28 slideably disposed in channel 14 for longitudinal movement with respect to the beam . the depth rod is employed for measuring the depth of an opening as is well known to those skilled in the art . thumb screw 30 is mounted on the slider frame and engageable with the beam for locking the slider to the beam . a dial indicator is mounted on the slider frame and has contained within it , the means ( not shown ) for engaging rack 16 so that needle 36 is rotated by means of center pinion 38 as the slider is moved along the beam . as the slider is moved toward the right , as viewed in fig1 the needle rotates in the clockwise direction . the dial indicator also has a bezel 34 which can be rotated with respect to the frame and carries indicia on annular scale or dial face 36 for indicating the slider being moved each 0 . 001 inch along the beam . the scale has a zero reading at 38 . the frame carries a second jaw 40 having a linear outside edge 42 which cooperates with jaw 18 in measuring the inside dimension of an opening in a workpiece ( not shown ). the frame also carries a second , longer jaw 44 having an inside linear edge 46 so that a workpiece can be disposed between jaws 20 and 44 for measuring an outside dimension of the workpiece . linearly arranged indicia on scale 48 is mounted along the lower edge of the beam , as illustrated in fig1 and begins with the base figure of &# 34 ; 0 &# 34 ; at the left end of the scale . a dimension - measuring marker 50 on jaw 44 is disposed and adjacent the &# 34 ; 0 &# 34 ; location when the inside linear edges of jaws 20 and 44 are in abutment . the dial indicator is adjusted so that needle 36 is adjacent the &# 34 ; 0 &# 34 ; figure on the dial face . a thumb - operated member 52 provides means for rolling the slider frame along the beam to either open or close the jaws depending upon the nature of the workpiece being measured . in conventional practice , either the workpiece is placed between jaws 20 and 44 , or jaws 18 or 40 are placed inside the workpiece . the distance between the two jaws is then measured and read directly from scale 48 and the face of the dial indicator . fig4 illustrates a pair of feeler arms 54 and 56 which may be clamped on jaws 20 and 44 for measuring a location such as at &# 34 ; a &# 34 ; in fig1 a location normally inaccessible for measuring with a conventional dial caliper . feeler arms 54 and 56 are identical but are mounted in a right and left hand relationship on jaws 20 and 44 . fig2 illustrates threaded fastener means 60 used on a feeler arm to clamp it to jaw 44 . several threaded fasteners may be employed . the lower arms 54 and 56 carry a pair of inside contact points 62 and 64 for measuring the outside dimension of a workpiece , and a second pair of contact members 66 and 68 for measuring the inside dimension of a workpiece , such as 70 shown in fig1 . fig3 illustrates the manner in which feeler members 62 and 64 are employed for measuring groove 72 of a workpiece 74 . the problem with employing arms 54 and 56 when they are mounted on jaws 20 and 44 is that when feeler members 62 and 64 are in abutment , they define a new base reference location which deviates from the &# 34 ; 0 &# 34 ; base location of scale 48 . to compensate for this deviation , linearly spaced indicia or scale 76 is formed along depth rod 28 and may be either in the form of a tape or permanently marked on the depth rod in the conventional spacing but reading in reverse , that is , from right to left as opposed to left to right of scale 48 . indicator 80 is slideably mounted on the beam and has a pointer 82 which is lined up against a selected measurement on scale 76 . thumb screw 84 provides means for clamping the indicator in a particular position along the beam . in use , when the accessory arms are being mounted on jaws 20 and 44 , contact points 62 and 64 are disposed in abutment with one another . indicator 80 is then moved along the beam so that pointer 82 is adjacent the &# 34 ; 0 &# 34 ; marking on the depth rod scale to indicate the new base reference reading . the bezel of the dial indicator is also rotated so that the &# 34 ; 0 &# 34 ; reading is adjacent needle 36 . the workpiece is then disposed between contact points 62 and 64 . in so doing , the jaws are spaced thereby moving the depth rod along the beam toward the right to a position in which scale 76 directly reads the outside dimension of the workpiece according to the position of pointer 82 and the dial indicator . the same procedure can be employed by mounting appropriate accessory arms on jaws 18 and 40 for measuring other locations that are normally difficult to measure with the conventional jaws . the preferred embodiment of the invention eliminates lengthy and cumbersome calculations for compensating for the deviation from the normal &# 34 ; 0 &# 34 ; reading by providing means for directly reading the actual dimension of a workpiece without calculating the deviation caused by accessory arms 54 and 56 . the deviation from the normal zero is always factored in when the dimension of a workpiece is being measured .