What: Achieving symmetry through repeated measurement and tools, not the eye alone.
Why: The eye deceives; measuring grants the precision that separates the refined from the acceptable.
How: Use a template or measuring jig, measure before each unit, and compare both sides on a reference axis.
Common mistake: Relying on visual estimation, so units drift out of true gradually.
Exercise: Execute three identical units from one template, then measure the differences between them.