Tamper-resistant or security fasteners are often used to prevent theft, vandalism, and unauthorized entry. Several types of threaded fasteners considered to be tamper-proof are designed to be non-removable or require a special tool to remove them. Some of these head styles are limited in their maximum size. Spanner heads, for example, are not normally made for screw sizes larger than ¼ in. And higher-quality tamper-proof fasteners have hardened heads, making it difficult to form a new drive slot with a file. In addition, many of the removable tamper-proof heads require special drive tools.