What Is Mul-T-Lock?
Mul-T-Lock is an internationally distributed line of high security lock cylinders distinguished by the use of a telescoping "pin-within-a pin" tumbler design and horizontally-oriented "dimple" keys.
The locks are UL-listed and are hardened against forceful attack. Most cylinders have 5 tumblers. Each tumbler contains an inner and an outer pin, giving a total of 10 independently keyed elements in a 5 tumbler lock, There are also versions that include additional secondary security elements, including side pins
Mul-T-Lock keyways are are oriented horizontally in the plug. Keys are double sided as a user convenience and can be inserted either way, although only the top of the key actually interacts with the pins.
Credit to Matt Blaze