What is an extension?

An extension is a small file that changes how your computer operates. For example, the QuickTime extension gives your computer the capability to play digitized video and audio files. Unlike a control panel, you typically do not have any direct interaction with an extension. An extension makes the appropriate changes when you start your computer and remains active until you shut down your computer. Extensions must be loaded into memory during computer startup. Therefore, when you place (or install) a new extension into your extension folder. it has no effect until you restart your Macintosh. Extensions with the letters "lib" in them are typically code libraries that may be used by multiple applications or system components. Turning off extensions or libraries may have unexpected effects. In general, you shouldn't turn off any extensions unless you know for certain what they're used for.

The contents of your Extensions folder may vary depending on the type of computer you have. You might also have extensions that were added by third-party applications. You can use the Extensions Manager control panel to turn off any extension you don't need. However, Extensions Manager will only list those files that have an INIT resource, or files that are set to be listed regardless of whether they have INIT code in them or not. In other words, Extensions Manager does not show all extensions and control panels.

The Extensions folder inside the System 7.x and 8.x System Folder contains "system extensions" (previously known as "INITs"), and miscellaneous software additions to your Macintosh computer, such as printer drivers and network drivers. The Extensions folder is the first place the System looks when loading system extensions at startup.