The Bishop-33 opening (3三角 san-san kaku) is an opening characterized by moving the bishop to the 33 square early while leaving the bishop diagonal opening allowing for an early bishop trade. The opening is flexible in that it can lead to an Opposing Rook (Ranging Rook) position as well as a Static Rook position with or without a bishop trade.
The most common variation and the one used by some professional players is the Fourth Move Bishop-33 opening (４手目３三角戦法 yon-teme san-san kaku senpō). When used by Black, this is known as Third Move Bishop-77 opening (３手目７七角戦法 san-teme nana-nana kaku senpō).
1. P-76 P-34, 2. P-26.
2...B-33. White's bishop moves to the 33 square on the fourth move of the game keeping their bishop diagonal open.
This move invites Black to capture the bishop which is only defended by White's left knight.