DirectX (or Direct X, sometimes abbreviated as DX) is a Application Programming Interface (API) used by almost all games running under Windows and on the different Xboxes. The interface provides low level access to the functions of modern 3D and 2D accelerated Graphics Cards.
All versions of the Tomb Raider Games that were released for Windows PCs use Direct X with the notable exception of the first Tomb Raider, which was written for DOS and used proprietary interfaces like Glide on the Voodoo range of Graphics Cards.
Before the rise of Windows to dominance over the PC marked, most games were written for the older DOS operating system, where access to the hardware was always possible as there were no protective measures in place to prevent programmes from manipulating any resources installed in the machine. The same is true for the Windows versions up to 3.11, which were basically only GUIs for DOS.
With the release of Windows 95 access to the hardware was made more difficult to prevent misbehaving programmes from freezing the PC. Applications now had to access hardware functions through well defined interfaces and drivers. One of these interfaces was Direct X. It provided access to graphics and sound hardware.
In a way this also helped the programmers as they did not have to write code specific to all the different video cards that were released to the marked, increasing the compatibility immensely. Direct X was adopted quite fast by the industry as well as the end users.
The first release was DirectX 1 for Windows 95. The last available version for Win 95 was DX 8.0a. For Windows 98, ME, and 2000, DX 9.0c is the last version available, for Windows XP it is 9.29. Windows Vista and 7 are restricted to DX 11, Windows 8.1 can use up to DX 11.2. The most recent version, DirectX 12, is only available for Windows 10.
Problems with Direct X
If there are problems with older games on newer PCs that run a higher version of DirectX than the game's manual specifies, one can try to install the Direct X Version that came with the game. Most of the times the newer versions of DirectX include all functions of older releases with some exceptions.
On newer Windowses, starting with Windows XP and even more so for Vista and the following incarnations, using the Compatibility Mode provided with these systems do - in most cases - work better, though, with the added benefit of not introducing old and unsupported code into these new environments.
Legend and Shader Model 2.0
There is an option in Lara Croft Tomb Raider: Legend that turns on the then new DirectX 9 Shader Model 2.0. It gives the game a more modern, Xbox 360 like look, but also crashes in specific locations. To bypass these problems, switch to the normal rendering mode (turn off "next gen" in the settings), load the last check point and play up until the next. Then quit the game and turn on "next gen" again. Repeat if necessary.