Door knockers are fixtures attached to the front door of a dwelling, intended to save the knuckles of people who might come knocking for admittance. They consist of two parts, a plate that is fixed to the door and an attached hinged piece, often a loop, that can be lifted and rapped against the plate. Door knockers can vary in size from the tiny little metal fixtures, often incorporating a peephole to see who is outside, that are suitable for apartments, to large, lavish brass displays featuring a lion's head holding the knocking ring.
Does your door need a knocker? If you already have a door bell, door knockers can be redundant, but most people want a door knocker for its looks rather than its function, so it's really a matter of taste. Door knockers are typically metal for durability, but this can range from the most common brass fixture to iron, pewter and stainless steel.
You really won't be able to tell how your door knocker will sound until it is affixed to your door, but consider the composition of your door. Most houses have hollow metal doors, and a large plate with a huge knocker will make booming echoes if struck. If you have a solid wood door, this is less of an issue.
You can find door knockers to complement any decor. If your home is a mini-mansion, you can safely go for the baronial look. A lion's head holding the brass ring, or a griffin, for example, would not look out of place. Think heraldic. On the other hand, if you live in a Colonial era farmhouse, a large lion's head will look ridiculous. Go for a simple ring.
You can also use door knockers to announce your affiliations or hobbies. Do you collect dragons? Attend gatherings of Wizard of Oz or Star Trek enthusiasts? Believe in fairies? You can find door knockers that proclaim your particular eccentricity to the entire neighborhood.