I'm old enough to remember when Wang cane out with an electronic calculator (calculation 'brains' in the next room hardwired to keypads / readouts on a number of desks). Everybody had a slide rule. The secret of how they work is logarithms: add the logarithms of two numbers together and you get the logarithim of the number you'd get if you'd multiplied them. The distances between the lines are the logarighm of each number printed above. The part that slides (or rotates - there were round slide rules as well) lets you add (or subtract) logarithmic distances and read off the result.
It's been at least 50 years since I've laid hands on one. One of the guys I worked with in software had a base-16 ('hexadecimal') one, but I think that was just a novelty item. The rest of us had pocket calcs that could convert bases (AKA 'hexalators').
Also a slide rule will get you significant digits but you need to figure the order (the number of zeroes) on your own. Just getting comfortable with that puts you ahead of most folks numeracy-wise. Sure wish someone had taught seven year old me about this.