bell guard

The bell guard is the part of a fencing weapon that protects your weapon hand.
 

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The "(" is the bell guard.
 

The bell guard varies in size for the three modern weapons...

In foil, the guard is pretty small--doesn't really even cover all of the hand.

In epee, the bell guard is about twice the size, as the hand is a perfectly legal target.  And, being closest to your opponent, it is aimed for quite frequently.

The sabre bell guard is about the same size as the epee one, but it actually curves around and attaches to the end of the weapon.  This is to protect the top of your hand as well as the front as sabre is a slashing weapon (making the hand a lot easier to hit).

The bell guard plays varying roles among the three weapons.

In the case of foil, the bell guard is largely useless, since your hand is not target area anyway. It serves mostly to keep your opponent's blade from smashing into your fingers if his blade slides down yours.

In the case of epée, the bell guard is quite important as it serves as a shield for your hand and (assuming you've got a decent stance) forearm, both of which are target area. The guard is also large enough to use to parry with, in a pinch. In some cases you can tell how much use an epée has seen by examining its guard. Relatively new weapons will look like glossy metal with many scratches, whereas old weapons will have what is virtually a matte finish.

In the case of sabre (and I apologize if this is inaccurate, I am not a sabreur) the guard helps to protect the arms, which are target, and plays a large role in defending the four line, since that is the area where the guard curves down across the hand to the pommel.