A standard door key allows use of a lock to open a door. It has a simple but important function that is based on its good design.
Door keys are typically made of steel which can be cut precisely with machinery - an important factor in the use of the key. The lock and key work hand in hand, so the right key needs to go into the right lock for the mechanism to work. This also means that the key must be cut precisely to ensure it works in the lock. The use of steel or similar metals is also important for the key's durability. Keys need to withstand forces such as twisting to avoid bending or snapping, friction to prevent premature wear and tear, and impact as it's a portable object that may be dropped or hit against other objects, without chipping or cracking.
The shape of a blade of a key is very much dictated by the lock that the key is intended for. However, there are varying shapes of the head of the key, from circular to hexagonal or a completely custom shape. The head of the key not only gives the user something to grip while using the key, but the different shapes can also help users to differentiate between multiple keys. A key head also typically has a hole that allows you to attach the key to a keyring.
The key is made to fit an average hand, so people can easily use the key without it being too big and awkward to handle with or too small and fiddly - they are made so that they are easy to grip between two fingers. Along with the material choice, the size of the key means that it is lightweight and compact, therefore easily portable and easy to store in a bag or a pocket.
