Cutting a physical auto key simply requires the locksmith to find the corresponding "blank" key that matches the shape and groove pattern of the original key, and then copy the pattern of the key's teeth on to the blank using a grinder to remove excess metal. Some key-cutting machines measure and duplicate keys automatically, while others require the operator to manually guide the grinding wheel to match the original and duplicate key.
For older cars produced before the late 1990s, physical key-cutting is usually all that is needed to produce a working duplicate key. However, some cars use radio frequency transponders inside their keys to ensure that only the original key can start the vehicle. Some vehicles produced after 2000 may include electronic "smart" keys that the car can read from a distance, allowing the operator to start the vehicle as long as the key is present inside the car. These types of keys typically require the owner to return to the dealership, as only the car's manufacturer can produce a key properly programmed to interface with the car's security system.
If an automotive key is lost, a locksmith may still be able to produce a replacement by examining the car's lock. This can be a time-consuming process, and is typically much more expensive than simply producing a spare key from an existing one.