The key type is similar to the string type, in that it can also hold any string. However, it is meant to hold strings that have the format of an Universally Unique Identifier (UUID). The format of an UUID is:

• 8 hexadecimal characters.
• A dash.
• Three groups of 4 hexadecimal characters, separated by dashes.
• A dash.
• 12 hexadecimal characters.

For example, 5748decc-f629-461c-9a36-a35a221fe21f is the UUID for TEXTURE_BLANK. The hexadecimal characters are output in lower case by all functions that output them (such as llGenerateKey), and are accepted in either case by all functions that accept them (such as llGetAgentInfo).

Type conversion between string and key happens automatically in most cases. One exception is the string concatenation operator (+). When concatenating a key to a string, the key has to be typecast to string first by prefixing it with (string). See examples below.

The Mono virtual machine stores extra data in key variables that helps the interpreter run faster, at the cost of script memory. This means that key variables take more space than string variables.

Another characteristic of the key type is that when comparing two key typed values, they are compared as strings. This means that if they differ only in case, even when they represent the same UUID, they won't be considered equal.

Most avatar UUIDs follow the version 4 UUID format (that is, the first digit of the second four-digit group is a 4, and the first digit of the third four-digit group is either 8, 9, a, or b). However, some very old avatar UUIDs (2002 and early 2003, for example Governor Linden) didn't follow this convention.

llGetOwner returns a key, and llOwnerSay expects a string. The following examples illustrate how automatic conversion from key to string happens:

llOwnerSay(llGetOwner()); // works
llOwnerSay("Your key is " + llGetOwner()); // FAILS - concatenation needs typecast
llOwnerSay("Your key is " + (string)llGetOwner()); // works
string OwnerKeyAsString = llGetOwner(); // works

llUnSit accepts a parameter of type key. The following examples illustrate how automatic conversion from string to key happens:

llUnSit("3d6181b0-6a4b-97ef-18d8-722652995cf1"); // works
key Texture = TEXTURE_BLANK; // works (TEXTURE_BLANK is a string constant)