The second place that you are most likely to find a ferrule is on a
fishing rod of at least
two pieces. The ferrule in this case is the
tip of one
segment of the rod and the
bottom of the other. The bottom end is
hollowed out and has a bit of a
flair to it. The tip end is designed to fit
snuggly into the other end. Not surprisingly, the hollowed out part is called the
female side, and the other part is called the
male part.
In a fishing rod, the ferrules turns out to be the place where the flex of the rod is "disturbed". A single piece rod flexes continually from top to bottom, depending on the taper of the rod, etc. Where there is a ferrule, the flex of the rod breaks down a bit because it is not one piece.
Most rods are two piece varieties, and have one ferrule. However, there are rods designed to be carried in backpacks and other smaller spaces. These rods will have three, four , or even five segments, and an accompanying number of ferrules.