1. Both the upside and the underside of the base are datum surfaces. 

2. The slider has two jaws on its left side and right side. The jaw on the slider's left is for the bottom measuring face. The jaw on the slider's right is for the upside measuring face. (When the base is on the inspection surface plate, the surface of the plate is regarded as the extension of bottom measuring face.) 

3. In case the inspection surface plate is unavailable, the upside of the base can be used as an inspection surface plate. 

4. Measurement can still be done with the slider's right jaw.

5. The range of application can be widened with some attachments.