Usually this comes after White opens with e4, Black replies with e6 followed by White’s d4 and Black plays d5. We now have here a couple of options, either push on to e5 or e4 x d5.
[Incidentally, if White opens with d4, Black can still play e6 with the idea of playing the Dutch opening, but we will leave that for another day.]
First the Advanced variation
Black now has the choice to castle long or short after reviewing the position.
Second is the Exchange Variation
White has bought the Queen out too early, it's Bishop and Knight are sadly still on the back row.
