Step meansurement is not the same as depth measurement. Depth (of a hole, typicaly) is measured useing the 'depth blade' or 'depth bar' that extends from the far end of the arm (or bar) of the caliper, with the end of the bar forming the reference face. Although step heights can also be measured in this manner, it is difficult to obtain accurage measurements because it is difficult to hold the caliper perpendicular to the surface(s) being measured. If the caliper is designed for step measurement, in the ideal case of a step between two flat, parallel surfaces with a perpendicular surface between them, the head of the caliper can be rested on the lower surface and the back of the arm can be placed flat against the face of the step, thus assuring perpendicularity.

Also, try googling 'caliper + inside + outside + depth + step'

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Note added at 13 hrs 41 mins (2004-12-15 09:40:36 GMT)
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... and the inevitable typo: accurate i.p.o. accurage