The Musketeer Blade (AKA Double Wide Epee, Extra Wide Epee, Theatrical Epee) is a triangular blade similar at a distance to a sport fencing epee blade, but is actually made on a very different die and is absolutely unusable in the olympic sport of fencing. These are the best choice for theatre productions because the triangular design allows the blade to be both relatively light and durable. It's roots are from the deadly court swords of the 1600's.
A word of caution: Some unscrupulous vendors sell normal sport fencing epees as Musketeer epees, calling them double wides or extra wides. These sport blades have the sport-legal maximum 24mm wide fortes, but still use the deep, sharp "V" sport fencing cross section rather than the shallow cross section of the Musketeer blade. They will not give satisfactory performance in non-sport fencing arenas.