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Mac OS X Panther (version 10.3) is the fourth major release of Mac OS X, Apple’s desktop and server operating system. It followed Mac OS X v10.2 "Jaguar" and preceded Mac OS X Tiger (version 10.4). Apple released Panther on October 24, 2003.
Since a New World ROM is required for Mac OS X Panther, certain older computers (such as beige Power Mac G3s and “Wall Street” PowerBook G3s) are unable to run Panther by default.

The system requirements
PowerPC G3, G4, or G5 processor (at least 233 MHz)
Built-in USB (indicative of a New World ROM being present)
At least 128 MB of RAM (512 MB recommended, 96 MB supported unofficially)
At least 1.5 GB of available hard disk space
Panther still supported the Classic environment fully for running older Mac OS 9 applications but made Classic application windows double-buffered, which interfered with some applications expecting to directly draw to the screen.

New and changed features
Apple advertised that Mac OS X Panther had 150+ new features, including:
Finder—Updated with a brushed-metal interface, a new live search engine, customizable Sidebar, secure deletion, File labels and Zip support built in. The Finder icon was also changed.
Fast User Switching—Allows a user to remain logged in while another user logs in
Exposé—Helps the user manage windows by showing them all as thumbnails.
TextEdit – TextEdit now is also compatible with Microsoft Word (.doc) documents.
Xcode developer tools—Faster compile times with gcc 3.3.
Preview—Increased speed with PDF rendering
QuickTime—Now supports the Pixlet high definition video codec
Font Book—Font Book, Apple's font manager, simplifies viewing character maps and installing fonts. It also allows the user to sort fonts into categories.
FileVault—On the fly encryption and decryption of a user’s home folder
iChat AV—The new version of iChat. Now with built-in Audio- and video-conferencing.
X11—X11 is built into Panther
Safari—New browser made to replace Internet Explorer for Mac, developed when the contract between Apple and Microsoft ended. Internet Explorer for Mac was still available but not supported by Microsoft or Apple. Safari 1.0 was included in an update in Jaguar but was used as the default browser in Panther.