Black must decide whether he will take back with the c pawn : 5...cxd6 or with the e pawn : 5...exd6.

In the first case, Black will have a majority of pawns in the center, which may be useful for a later e5 push.

In the second case, the game will be more positional, relying more on the pieces placements than on the pawns fight.

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'1.e4 Nf6 2.e5 Nd5 3.d4 d6 4.c4 Nb6 5.exd6'

Wikipedia has related information at Alekhine Defence.