Architecture & practice·Complexity and how it accumulates
the VAT rate changed and you had to find it in eleven places, and missed one.
Don't repeat yourself
Also calledDRY, single source of truth for knowledge
Every piece of knowledge should have one authoritative representation. The original framing is about knowledge, not text: two identical-looking blocks that encode different decisions are not a violation, and two very different-looking pieces of code that encode the same rule are. Misapplied at the level of characters it produces the worst abstractions in most codebases, which is why the counter-slogans exist.