Every serious chess database tags games with an ECO code — something like B90 or D30 — and most players learn to recognise a handful of codes for their own openings without ever understanding the system behind them. Once you do understand it, the code itself becomes useful shorthand.
The five letters
ECO splits all chess openings into five broad categories. A covers flank openings and various 1.d4 systems that don’t fit elsewhere. B covers semi-open games after 1.e4, excluding the French. C covers the French Defence and open games beginning 1.e4 e5. D covers closed games built on 1.d4 d5. E covers Indian systems built on 1.d4 Nf6 without an early d5.
Within each letter, the two digits after it (00–99) get more specific as theory goes deeper — a Sicilian might start as B20 in its most general form and narrow down to something like B96 for a very specific, well-analysed line.
Why this matters practically
When you’re reviewing a database of master games or looking at your own game history, the ECO code tells you instantly what kind of structure you’re looking at before you’ve read a single move. B90 tells you it’s a Najdorf Sicilian. D30-something tells you it’s a Queen’s Gambit Declined structure. That’s enough to activate the right pattern-recognition before you even start calculating — which is exactly why every opening tutorial on this site is labelled with its code.
