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The coppersmith's shop had an heirloom heavy ruler, its gradations as fine as a hair, measuring window frames with great precision. But the heavy ruler was long and cumbersome; when the apprentice carried it up to the roof, one trip left him gasping for breath.

The shopkeeper wanted to give each apprentice a light ruler. The first version halved the scale markings, making it easy to carry, but when making fine window lattices it was off by half a finger, and only after installation was the drafty gap discovered.

The shopkeeper's first fix was to carve all the markings back on. The light ruler became heavy again and lost its meaning. The apprentices complained: if accuracy required carrying the ancestral heavy ruler, why bother with a new one?

The old coppersmith did not copy it wholesale. He first looked at which measurements needed extreme precision and which only needed rough accuracy: the width of a door opening could tolerate a little deviation, the depth of a tenon could not be off, and a decorative border line would be conspicuous even if off by a hair.

The second version of the light ruler kept fine graduations at key positions and changed insensitive intervals to coarse marks. The first trial still had errors, so the old coppersmith re-densified the sections that most often went wrong, while keeping the rest light and portable.

After several rounds, the light ruler was no longer identical to the heavy ruler, yet it could handle most work. The apprentice climbed roofs faster, important interfaces could still be measured accurately, and only extremely rare fine carving work called for the ancestral heavy ruler.

The shopkeeper understood: shrinking a precision instrument is not about casually cutting away details. You must know which precisions affect the result and which errors are tolerable, then use a lighter representation to preserve the main capabilities.

Later the shop hung both the light ruler and the heavy ruler on the wall. For most work they used the light ruler: fast, with controllable error. For fine palace carving, they still brought out the heavy ruler. The shopkeeper no longer sought to make the light ruler equal to the ancestral ruler everywhere, but let different precisions serve different scenarios. Lightness has value, provided you know where the loss falls. The old coppersmith also told the apprentices that the light ruler was not meant to show off lightness, but to make more scenarios affordable and portable. As long as the error is seen, controlled, and placed in the right task, becoming lighter is not a step backward but another kind of practicality. Later the apprentice carried the light ruler onto the roof, knowing where the weight he saved came from, and knowing when it could not be saved. This saying was later written into the newcomers' handbook, reminding everyone to look at the on-site cost first before talking about saving effort. And because of this reminder, when similar difficulties arose later, people would not rush to apply old methods. They learned to first ask which step bears the risk, then decide whether to let go, tighten, or rearrange the process.