**Mathematica Notebooks**: Analyzing even the simplest physical situations in general relativity can sometimes require lengthy
algebra, or lead to differential equations that do not have elementary, closed-form solutions. To help with this Mathematica Notebooks are provided which do some of the standard algebra and numerically solve some of the most important differential equations.

**Web Supplements**: Occasionally a longer than usual calculation is needed to derive or back up a result that has a simple physical motivation. Such calculations can divert the exposition of the central physical points. Readers approaching the subject for the first time may be willing to accept the result and return later to its justification. In a traditional text such calculations might have been relegated to appendices. Here they are presented as a collection of "Web Supplements" to help keep the text to a manageable size.

**Color Images**: A number of images in Gravity, especially astronomical photographs, are more vivid, clearer, and more persuasive in their
original color versions. Economic realities prohibit the publication of a text at this level in color. The color versions of these images
are presented here.

**Web Links**: Information about many topics in this text can be found on the Internet. (Try searching under "Global Positioning System" or "Gravitational Lensing" for example.) The author has not attempted to provide a comprehensive list of sites relevant to gravitational physics. This list contains a few links that are directly relevant
to the exposition in the text.