Chess Castling Move
Castling is a special move that helps hide the King in the corner. It is an unusual move, because you move 2 pieces at once: the King and a Rook. Even though you move both pieces, it is considered to be a single chess move.
Castling Move Basics
How to castle: The way that you castle is to move the King 2 squares towards the Rook, then move the Rook over the King to sit right next to the King. The King must move exactly 2 squares, not 1 or 3 squares.
How to castle correctly in tournaments: The strictly correct way to castle on a physical board is to use only one hand, pick up the King first and move it 2 squares, then pick up the Rook and jump it over. This is the proper etiquette in tournament chess, although in beginner tournaments it is rarely enforced. A common way that beginners castle is to use two hands, one each for the King and Rook. Another way seen with beginners is to to pick up the Rook first and then the King second. In strict, formal chess, picking up the Rook first would be a "touch-move" on the Rook, requiring a Rook-only move.
How to enter a castling move in a computer interface: In the chess player interface, the way to do a castling move is simply to select the King, and then select the square 2 squares away (towards the Rook), or to drag-and-drop the King 2 squares towards the Rook. The computer interface will then automatically know that you want to castle on that side, and it will move the Rook as well; you don't need to move the Rook yourself.
Castling not possible: Castling is not always allowed in chess. If castling is not allowed, the move will be prevented by the computer. The many possible reasons why castling might be disallowed are shown below.
Basic Castling Restrictions
Cannot 3-square castle on the Queen-side: You can only castle by moving the King 2 squares toward the Rook, then flipping the Rook to the other side of the King. On the Queenside, it looks like you could move the King 3 squares towards the Rook, instead of 2 squares, but this is not allowed.