Not all slide rules have to be straight as evidenced by this device here. This particular one is a promotional giveaway from C. W. Brabender instruments and comes to my collection by way of my late father- in- law. This seems to be a Takeda "Concise" rule which had a custom logo printed on the front.
The circular rule wraps the logarithmic scales around in a circle instead of in a straight line, as is the case with a "classic" rule. This allows the devices to be made fairly small in size so they'll fit in a pocket. They were an alternative to the small straight pocket rules . This one measures about 3.15 inches in diameter.
The scales on this model are D, C, CI, A, and K, from the outside to the inside.
The reverse carries a lengthy equivalence table with the proper setting
for the rule for each. For instance, to convert from miles to kilometers,
one sets 87 on the C scale opposite 140 on the D, then reads miles on
C and gets kilometers on D. For these functions, the circular rule is
very nice in that one never needs to reset the slider.