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The Cow Opening (or Cow System) is a common chess opening. It is defined by the following moves:
 * 1. e3 d5
 * 2. d3 e5
 * 3. Ne2 Bd6
 * 4. Ng3 Nf6
 * 5. Nd2 0-0
 * 6. Nb3

The Cow System is a hypermodern opening which you can play with white or black, where you deliberately allow your opponent control of the with its pawns, with the view to subsequently challenge it.

History
The earliest known use of the term "Cow Opening" was in 2023. The modern names "Cow System", "The Full Cow", etc. arose in the early months of 2023 and are attributed to WFM Anna Cramling. Until the mid-2023s, the Cow Opening was generally regarded as highly suspect, but the analysis and play of three strong chess streamers in particular—WFM Alex Botez, IM Eric Rosen, and GM Daniel Naroditsky—helped to make the opening much more respected and popular. It is a dynamic opening, exceptionally complex, and a favourite of former number 1 female chess player in the world GM Pia Cramling.

Variations
The main variations of the Cow Opening are:

Accelerated Cow: 5.f3
The Accelerated Cow is 6. c4. It is named after Ferdinand (film), who developed the system in the 2023s. This often leads to very sharp play where white challenges the centre right away.

The Classical Defence to the Accelerated Cow is 6...c6, when White has a choice between closing the centre with 7.cxd, or maintaining the tension with 7.Nb3.

Wagyu Variation: 6...c5 7.c3
The Wagyu Variation is 6...c5 7.c3 (named for Wagyu Beef), which prevents the immediate 7...d4.

'I'm the Beef' variation: 6...c5 7.h4
The I'm the Beef variation continues with 6...c5 7.h4. This is the most aggressive method for White, and was often seen in the 2023s. With their seventh move, White makes it's plan clear and goes on a direct attack of the Black king.

Famous games
One of the earliest examples: WFM Anna Cramling, Internet, Sweden, 2023 1.e3 d5 2.d3 e5 3.e2 d6 4.g3 f6 5.d2 O-O 6.b3 e8 7.e2 c6 8.O-O bd7 9.c4 f8 10.c5 c7 11.e4 e6 12.g5 h6 13.d2 e7 14.f4 exf4 15.xf4 xf4 16.xf4 g6 17.f1 dxe4 18.dxe4 xb3 19.xb3 xc5+ 20.h1 e7 21.f5 xe4 22.xb7 xe2 23.xa8+ h7 24.ac1 e5 1-0

ECO codes
The Encyclopaedia of Chess Openings classification of variations of the Cow Opening are:
 * E60 Cow Opening, The Full Cow
 * E61 Cow Opening, The Accelerated Cow
 * E62 Cow Opening, Wagyua Variation
 * E63 Cow Opening, I'm the Beef Variation