D · fixed
C · slides ⟷
D · fixed
line up:

This is a real slide rule — it computes nothing for you. Grab the middle C strip and drag it so its 2 sits directly under D's 4. Then look down the strip to C's 4: it now rests under D's 8. That is 2 × 4 = 8 — you read it straight off the wood. Slide the red glass cursor to help your eye hop between scales. (The buttons just pre-slide the strip for you; the answer is always wherever the numbers line up.)