Early in my writing career, I had an assignment to follow around a mohel--the guy who does ritual circumcisions in the Jewish tradition. My subject learned the trade by watching his dad, a renowned figure in the field. One day, father told son he was ready to handle the tools himself. Why now, the son wanted to know. "Most students ask me how much to take off," the senior explained. "You asked me how much to leave on."

Apple faced a similar question when designing the MacBook Air, the subnotebook computer that goes on sale next week. The category--ultraportable laptops weighing less than four pounds--has been known for sharp compromises in price, performance and features, all in service of the high-tech equivalent of a crash diet. What to leave in and what to take off?

Certainly Apple has fulfilled its goals in terms of thinness. The Air is a lithe sheath of aluminum so slim that it can slide under my office door.  Packed inside the shell--which is three quarters of an inch at its thickest point, trailing off to a wispy 0.16 inches--is two gigabytes of memory, a bright 13.3-inch screen (lit by cutting-edge LED technology) and a full-size keyboard. This is a top-of-the-line array for a subnotebook. And, of course, it runs the Macintosh Leopard operating system, which you know, if you've seen the ads, is superior to Microsoft's competing Vista OS. (The commercials are right.) Did I mention that it's really skinny? When I slip it in the sleeve of my backpack where my six-pound MacBook Pro usually travels, the pocket still looks empty. Surely this is salve for the shoulders of anyone who springs the $1,799 it costs to buy.

The Air shines most, of course, when it's out in the open--on an airplane seatback tray, on a conference table, beside your latte in a Starbucks and on your lap when you're sprawled on the sofa. (Bonus: the Air doesn't run as hot as Apple's other laptops--it's actually possible to work for an hour with the device on your lap without the feeling that your fertility is at stake.) The gentle curves and the absence of protrusions make this an instant object of techno-lust, another notch in Apple's belt of design triumphs. Most importantly, its diminutive dimensions pretty much evaporate the eternal quandary of whether or not to take your computer along with you.

The compromise story is more complicated. Apple was unstinting in including an excellent keyboard with its great automatic backlighting feature, which radiates illumination in dim conditions. Its brain is the powerful Intel Core 2 Duo processor (though running at a lower clock speed that Apple offers in other laptops). It's got a built-in video camera for conferencing. The screen is big for a subnotebook, and quite bright. And the battery life is quite acceptable--I didn't have time for a definitive study but was getting only slightly less than the five hours per charge that Apple promises.

Also, the Air also breaks ground in being the first Apple computer to integrate some of the multi-touch technology introduced on the iPhone. Apple's smallest laptop also sports its biggest trackpad, the better to perform digital (in the old sense) tricks like the three-finger swipe (to flip through photos or to page back or forward on the browser), the two-finger scroll, the pinch-and-stretch (to resize Web pages) or the rotate maneuver (easier to perform than to describe). All are preferable alternatives than doing those common screen manipulations the way you used to do them.