. What is a contextual menu? How does it differ from the regular menu bar at the top of the computer screen?

A. Menus - lists of options, commands and controls - are a common part of operating systems, like Microsoft Windows and Apple Computer's Mac OS, that use a graphical interface and a pointing device like a mouse.

But menus are not limited to the top of the screen; often one will pop up wherever your mouse is pointing when you right-click. These hidden menus are called contextual menus because they appear in the context of what you are doing. They can provide an efficient way to execute common commands that might be buried in the program's menu bar.

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If you are working in Windows and want to change your wallpaper background, for example, rather than going from the Start menu to the Control Panel area and double-clicking on the Display icon to get to the wallpaper settings in the Display Properties box, you can simply right-click on the desktop and select Properties to bring the box up.

Many programs have built-in contextual menus related to common functions that can spare you from wasting time wading around in the menu bar looking for, say, the settings box for font or paragraph formats.

If you use a computer system that has a single-button mouse (as many Macintosh systems do), you will often call up a contextual menu by clicking on the screen while holding down the Control key on the keyboard.

In the most recent version of Microsoft Word for the Mac, control-clicking on a specific word in the document will call up a contextual menu of basic editing and formatting commands, plus a shortcut to the program's thesaurus and - if the word is misspelled - to the spelling-checker dictionary. Windows Word users can call up these contextual menus by right-clicking on a word.

Q. Why does a Web page's appearance differ depending on whether I am using Internet Explorer or Netscape Communicator?

A. Although they started out as simple electronic documents that could display hyperlinked text and photographs, Web pages have in the last decade grown much more sophisticated in both their design and construction. Web pages now can contain cascading style sheets, Flash animations, JavaScript and other coding that turns a page view into an interactive multimedia experience.

Not all Web browsers are designed the same way, and the appearance of some pages can be affected by which one you are using. Some versions of Netscape's browser, for example, tend to display fonts at smaller sizes than Microsoft's Internet Explorer does, which can affect a page's appearance.

There are other Web browsers, too, including Opera, Mozilla and Safari; each may display the same page slightly differently. There are also differences between the way Web browser programs for Windows interpret a Web page and how a browser on a Macintosh may render it.

To save time and money, many Web design shops optimize their pages to look best in one or two of the most commonly used browser programs. The Windows version of Internet Explorer is the most frequently used Web browser program (partly because of its integration within the Windows operating system), so most designers create Web pages that look best in that browser.

Q. Is there an easier way to take screen shots in Windows than by using the PrintScrn button and pasting it into the Paint program?

A. If you are tired of saving screen images the old-fashioned way - by creating files with your built-in Windows Paint program - you might want to consider screen-capture shareware.

Freeware programs like Gadwin PrintScreen (www.gadwin.com /printscreen) can take screen shots of the entire screen or just one window with a keystroke, and save the file into one of several common image formats. Similar programs can be found at shareware sites around the Web. J. D. BIERSDORFER

Circuits invites questions about computer-based technology, by e-mail to QandA@nytimes.com. This column will answer questions of general interest, but letters cannot be answered individually.

Circuits invites questions about computer-based technology, by e-mail to QandA@nytimes.com. This column will answer questions of general interest, but letters cannot be answered individually.