A taping knife or joint knife is a drywall

Drywall

Drywall, also known as plasterboard, wallboard or gypsum board is a panel made of gypsum plaster pressed between two thick sheets of paper...

 tool with a wide blade for spreading joint compound

Joint compound

Joint compound is a white substance similar to plaster used to seal joints between sheets of drywall, primarily in building construction. It is often referred to simply as mud.-Ready-mix lightweight joint compound:...

, also known as "mud". It can be used to spread mud over nail and screw indents in new drywall applications and is also used when using paper or fiberglass drywall tape to cover seams. Other common uses include patching holes, smoothing wall-coverings and creating specialty artistic wall finishes. Common sizes range from five to twelve inches wide. Spackle knives are a smaller version, used for patching small holes.

A right-angle joint knife allows one to apply joint compound to inside corners where walls meet. The handle is offset to allow clearance for fingers.