I'm curious why this particular shape of jaws on digital caliper like this one has.

enter image description here

I mean the tapered pointy ends of the outer and inner jaws. The different shapes of the inner jaws one having a step.

2 Answers 2


The jaws have a knife edge for measuring the distance between two holes, which is needed when measuring the distance between hole centres.

If the jaws didn't have a knife-edge tip, the width of the tip would hold the caliper away from the hole at two points and the measurement would be wrong.

enter image description here

  • A - knife edge on inside jaws for measuring inside holes
  • B - recess to give clearance and define measuring surface
  • C - ditto
  • D - rectangular edge for measuring flat (or convex) surfaces. This edge is usually lapped flat, an expensive precision operation that cannot be easily performed up against an internal corner, hence the recessed corner.
  • E - knife edge for measuring in holes (or similar surfaces that are concave in one dimension) or where access is limited.

The recesses for clearance are found on other tools where you need to prevent sharp corners (or burrs) on the object from contacting a radiussed inner corner on the measuring tool.

Note the recess in the corner of this engineer's square

enter image description here

There are a wide range of jaw shapes available for specialised purposes.

enter image description here Source - Mitutoyo

The most common jaw shape is probably a compromise that is useful in a wide range of situations.


The tapered ends in the shorter inside measure jaws allow for measurement of diameter of small holes, narrow grooves or small spaces between two other parts.

  • and why the step on one of the inner jaws?
    – axk
    May 5, 2017 at 8:22
  • axk, as was stated above the step B in the picture above limits the measuring length of the jaws allowing the instrument to be made with parallel measuring jaws of a defined length. This reduces the cost of manufacture and allows the instrument to be more easily calibrated or certified. It does allow an interior measurement of say a tube which might be slightly constricted (by the cutting process) at the open end. May 5, 2017 at 10:15

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