[alexjm]

I have a kitchen slide that I use for ratios in recipes. It's an old plastic one from Think Geek.

For example, I usually put 15 grams of coffee with 8 oz of water (please excuse the mixed units). To make a different amount, I align the 1.5 on the top rule with the 8 on the bottom rule to set the ratio. Then each number on the top rule (coffee in grams) matches the scaled value on the bottom rule (water in oz). The 6 on the bottom rule aligns with ~1.1 on the top, meaning I should brew my little six-ounce cup with 11g of coffee. In practice, I do this a lot with bread, but the "baker's percent" convention for writing bread recipes makes it a more complicated example.

Another way to use a kitchen slide rule is when scaling a recipe. Say I want to make 2/3 of a batch of cookies. I line up the 3 on top with the 2 on the bottom. Then for each ingredient, I find the recipe's quantity on top, and read off the scaled quantity on the bottom. This works better with recipes that use weights, to avoid awkward fractions or converting between units so you can subdivide.