Side Pawn Bishop-33.

Static rook, played by White · castles into Central House, on both sides · 横歩取り△３三角 yokofudori san-san kaku

The idea

This line is White’s. In the Side Pawn Capture, Black’s rook takes the pawn on 3d and threatens to trade bishops on 2b, which would win a gold or make a dragon. The bishop to 3c stops it, and en.wikipedia calls this the most common Side Pawn variation. Black is a pawn up and your rook is out early; ja.wikipedia counts the position as equal.

The form is named after Naitō Kunio, who first played it in a professional game in 1969, and the play that follows is called the Aerial Fight (空中戦法). In this main line your rook comes back to 8d, eyeing 2d; Black stops that with the rook to 2f, which also hits your knight, and you put the silver on 2b in front of it.

After Black drops a pawn on 8g, both kings settle in a Central House, the king one step up between two golds, and your king move has to come first. The moves are one opening guide’s from move one; a second gives the middle of the line with a reason for each move, and en.wikipedia’s diagrams match it, but the last five moves have the one source.

Played with the silver to 2b before the rook retreats, the line keeps the rook’s retreat to 8e open: that is Chūza’s Rook. Black’s 17th move has two answers with pages of their own, the Aono and Yūki lines, which leave the rook on 3d.

How it goes together

Both sides’ moves are shown, because this opening is an agreement rather than a system: one player cannot reach it alone, so a one-sided list of moves would be a fiction.

The plan once you are set up

• After Black’s rook takes the side pawn, play the bishop to 3c at once.
• Pull the rook back to 8d, where it eyes 2d.
• Answer Black’s rook on 2f with the silver to 2b.
• Move the king to 5b before anything else, then the right gold to 7b and the right silver to 6b.
• Swing the rook to 7d to hold Black’s pawns back.

What to watch for

• The king move. If the king is not yet on 5b, Black trades bishops on 3c and drops one on 2a to win material; with the king there, the gold to 3a answers it.
• Black’s pawn on 8g is there to stop a bishop trade on 8h followed by a bishop dropped on 4d. Without it, that combination is yours.
• Black has other 17th moves: the king to 5h (the Aono line) and the king to 6h (the Yūki line) leave the rook on 3d.

What it is played against

Black keeps the extra pawn, pulls the rook back to 3f and then 2f, drops a pawn on 8g and builds a Central House with the gold on 3h and the silver on 4h.

From the end of this line the source gives two ways on: the bishop to 7g, heading for a Silver Crown, or the pawn to 8f at once and an attack on the edge.