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A biscuit joiner uses a special horizontal blade that plunges into a piece of wood to create a slot for a biscuit. A biscuit is a football-shaped piece of compressed wood that helps join together two pieces of wood. To use a biscuit joiner, align the joiner on the piece of wood and push the handle forward. Make a similar cut in the second piece of wood at the place where the two will adjoin.
To use the biscuit joiner, place the two boards side by side and mark a line over both boards as to where you want the biscuits to go. Then align the joiner with the line on the boards and slowly push the joiner forward, creating the hole in the side of the board. I wrote an in depth article with pictures on how to use a biscuit joiner. Let me know if you have any questions. I can even take video showing the method.