QI'm shopping for a high-definition TV, but which ones are really HD? I thought a set had to display at least 720 by 1,280 pixels, but some fall short in one or both dimensions.

AThis reader has gotten spun around by the conflicting definitions of manufacturers, retailers and industry bodies.

At a minimum, a high-def set must display 720 lines of resolution across a wide screen with 16-by-9 proportions. Once or twice, though, I've seen less scrupulous (or perhaps less attentive) vendors and stores label digital TV sets "HD" that miss on one or both aspects.

That two-part description of HDness comes from the Consumer Electronics Association, an Arlington trade group. The Federal Communications Commission uses it, too. But although this definition (www.ce.org/shared_files/resources/95DTV Definitions.pdf) specifies a screen's relative width and rows of pixels (short for "picture elements," the tiny dots that make up an image), it says nothing about how many columns of pixels the set should display.

A different trade association, the Advanced Television Systems Committee, develops standards for digital-TV transmissions -- that is, not how they look on your screen -- and it counts horizontal as well as vertical pixels. One of its two main HD formats, 720p, provides 720 by 1,280 pixels (with every row redrawn 60 times a second, what's called "progressive scanning"). The other, 1080i, offers 1,080 by 1,920 pixels (here, odd and even rows of pixels take turns being redrawn, what's called "interlaced scanning").

So what should you believe? How about your own eyeballs? Small differences in resolution are unlikely to make as much of a difference as the overall quality of one HD set versus another. Go to a good electronics store, put the same HD program on the sets you're considering and see which one looks clearer to you.

-- Rob Pegoraro

Rob Pegoraro attempts to untangle computing conundrums and errant electronics each week. Send questions to The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or rob@twp.com.