A modification, also known as mod or plug-in, is a software file that changes some aspect of the game or adds new content, such as weapons, armor, or quests. Several official plug-ins exist for the Xbox 360 (see below), but modifications are overwhelmingly most common for the PC platform. Mods do not come pre-packed with the retail game. They are created by the game's fans, usually using the Elder Scrolls Construction Set for Oblivion. They can be downloaded from the internet, usually for free. Bethesda also offers official studio-built mods, which may be downloaded for a small payment.