A governing mantra in my life comes from my grandmother and my mother: “You’ll never notice it on a galloping horse.” An adage intended to ward off obsessive behaviors of perfectionists, it was often employed in sewing. It’s why I’m not afraid to duct-tape my hemlines in a pinch.

Unfortunately, that good-enough approach does not work when constructing stairs.

A single step should rise somewhere between 7 and 7.75 inches, so the math works like this: How far do you need to climb from finished floor to finished floor? For example, if you need to travel 133.25 inches, divide that number by 7, and you get 19.03. Well, obvisouly you can’t have 19.03 steps, so decide whether you want 19 steps or 20 steps. Start by dividing 133.25 by 19; you get 7.01, an acceptable rise, so you can have 19 steps that each rise 7.01 inches.

Or, if you’re me, each step will rise 7 inches, because the difference between 7 inches and 7.01 inches is so small that you would never notice it on a galloping horse.

Not so fast! We’ve got 19 steps to deal with, and if we are off by .01 on each step, by the time we get to the 19th step, we’ll be off by 0.19 inch.

According to building codes, step-rise can’t vary by more than 0.125 inch, and 0.19 inch is more than that. The one-hundredth matters. But how are you supposed to measure one-hundredth of an inch?

With a depth micrometer or depth mic. In the picture below, we are using a Starrett brand depth micrometer to position stair gauges (the octagonal brass nuts seen here) perfectly on a framing square.

On the one hand, this behavior does not come naturally to someone who duct-tapes her clothes together. On the other, for a colossal worrier like me, it is the most natural thing in the world. Stairways result in 4,000 fatal falls and 1 million injuries requiring hospital treatment each year, so it’s worth getting these details right to prevent galloping horses—and people—from tripping on uneven stairs.