Flank openings are all those openings which do not begin with 1. e4 or 1. d4.
Bird: 1. f4
In Bird's Opening White begins with 1. f4 and takes direct influence over the e5-square but at the same time weakens his king's position.
The English Opening starts with 1.c4. It is an important part of chess theory.
The Larsen Opening arises after 1.b3. It is named after the former world-class Danish player, Bent Larsen.
Reti: 1. Nf3 d5 2.c4
The Réti Opening begins with the moves 1.Nf3 d5 2.c4. White offers a pawn sacrifice to dislodge Black’s central pawn.