A skewer is an attack upon two pieces in a line and is similar to a pin.
In fact, a skewer is sometimes described as a "reverse pin". In a skewer, a valuable piece is attacked and forced to move to avoid being captured, leaving a less valuable piece behind it free for capture. Like pins, skewers can only be created by the long-range pieces (queen, rook and bishop).
Wikipedia Skewer page - General info and a few examples
Basic Tactics Lesson - The Skewer - Good lesson at Chessville.com
Chess Skewer Lesson - From Coastal Carolina University Chess Club
Predator at the Chessboard Lessons
Simple Skewers through the King - 13 page Lesson
Simple Skewers through the Queen - 9 page Lesson
Simple Skewers through Rooks - 5 page Lesson
Substituting the King to create a Skewer - 3 page Lesson
Other Substitutions Queens, Rooks, etc - 7 page Lesson
Checking the King into position - 10 page Lesson
Working with Multiple Checks - 11 page Lesson
Getting the Skewering Piece to the Skewering Square - 7 page lesson
Consolidating Excess Enemy Pieces - 10 page Lesson
Preventing the Middle Piece from Guarding the Target- 6 page Lesson
Skewers where Enemy Pieces are Under defended - 3 page Lesson
Breaking the Skewer -7 page Lesson
(Over 90 examples in the above lessons)
7 examples Of Skewers
Introduction to basic tactical themes- Pins and Skewers - 6 min Video
Rook Endgame Skewer - 2 min Video
More Work for Ken Regan?
1 day ago