Restless Knights
The Pin
A pin is when a piece is held in place (pinned) by an enemy piece. The pinned piece cannot move without exposing a more important piece behind it. The attacker, the pinned piece and the stronger piece behind it are all on the same line, whether the line is a file, rank or a diagonal. Pieces that can pin are queens, rooks and bishops, in other words all the long-range pieces.
Pins can be either absolute or relative. If the pinned piece is shielding the king, the pin is absolute, the pinned piece can absolutely not get off the line of attack. A relative pin is when the pinned piece is shielding some other piece than the king and is allowed to move off the line of attack, it's just not going to be a good idea.
Finally, a pin can be said to be partial, when the pinned piece can move along the line of attack, just not off the line. The queen can only be partially pinned.
Tips for using the board below: If you tap the f3-square, that'll take you to the next game. C3 will take you back to the previous one. If you tap e7, that will flip the board. Note that once you've flipped the board, the functions don't flip, so e7 becomes d2, f3 becomes c6 and c3 becomes f6. Many more squares have functions, I'll let you discover them for yourself if you want to.
A partial pin. The queen is not completely frozen, she can move vertically, although that won't of course save her. Note that this is also an absolute pin, she cannot move out of the line of attack.
A relative pin. The bishop can move, but it's a really bad idea!
An absolute pin. The knight couldn't move even if it wanted.
An absolute pin. The knight couldn't move even if it wanted.
An absolute pin. The bishop couldn't move even if it wanted.
An absolute pin. The rook couldn't move even if it wanted.