ECO C20 Explained: The King's Pawn Game and Its Offbeat Second Moves

If you have looked up an opening and seen a code like C20 next to its name, you have met the Encyclopaedia of Chess Openings classification. It divides every opening into five volumes, A to E, each with a hundred codes, 00 to 99. The system was built for the reference books of the same name, but it outlived them because it gives every line a short, stable address that means the same thing in every language.

C20 is the address for the King's Pawn Game: 1.e4 e5 followed by a second move for White that does not already have a code of its own. That last part is the key to understanding it. The popular second moves each have their own territory — 2.Nf3 leads into C40 to C99, which contains the Ruy Lopez, the Italian Game, the Scotch and the Petrov; 2.Nc3 is the Vienna Game, C25 to C29; 2.Bc4 is the Bishop's Opening, C23 and C24; 2.f4 is the King's Gambit, C30 to C39.

So C20 is, in effect, the code for everything else. That is why it contains such a strange mix: the calm 2.d3, known as the Leonardis Variation; 2.c3, the MacLeod Attack; 2.Ne2, the Alapin Opening; and the lines that are famous mostly as jokes, like 2.Ke2, the Bongcloud Attack, and 2.Qh5, the Wayward Queen Attack.

None of those moves is a serious attempt to win an advantage, and it would be a mistake to read the shared code as a judgement on them. The code tells you where a line sits in the tree, not how good it is. 2.d3 is a perfectly respectable way to reach a reversed Philidor structure; 2.Ke2 gives up the right to castle on move two for no reason at all. Both are C20.

The one C20 line you are genuinely likely to meet at club level is 2.Qh5. It threatens Qxf7# once the bishop reaches c4, and it catches a surprising number of players below about 1200. The correct response is calm: 2...Nc6 defends e5, and after 3.Bc4, 3...g6 or 3...Qe7 blunts the attack. Once the threat is parried, White's queen is simply exposed, and Black develops with tempo by hitting it.

2.d4 is a useful boundary case. It starts in C20, but as soon as the game continues 2...exd4 it becomes the Center Game or the Danish Gambit, with their own codes in C21 and C22. This happens constantly: a single move can carry a game from one ECO code into another, which is why the code on your game record is always the deepest named line the game reached, not the first.

The practical use of ECO codes is search. Opening names vary between sources and languages — the same line can be called three different things — but a code is unambiguous. If you want everything around 1.e4 e5 with an unusual second move, searching for C20 finds it in one step, which is exactly what most people typing "eco c20" into a search engine are trying to do.

If you are building a repertoire, C20 is worth ten minutes, not ten hours. Know how to meet 2.Qh5, know that 2.d3 and 2.c3 are harmless and let you develop normally, and spend the rest of your opening time on the lines you actually face.