Until now, ““subnotebooks,’’ as the smallest, lightest portable computers are called, have lagged far behind their larger cousins. Users resented the tiny screens and the crowded keyboards, which were 10 percent smaller than the standard laptop’s. Despite optimistic forecasts, only 345,000 of them sold in the United States last year (as compared with the 3.5 million laptops). With the ThinkPad 701C, IBM fits a full 11.5-inch keyboard and a 10.4-inch screen (measured diagonally) into a 9.7-inch, 4.5-pound package. Such specs translatein ““breakthrough technology,’’ says Kimball Brown of thecomputer market-research firm Dataquest. Consumers already seem to agree. IBM wouldn’t be specific, but a spokesman says the company has logged more advance orders than for any other personal computer in its history. Industry analysts say that the era of the ultraportable has finally arrived.

IBM’s clever new mini works like a sophisticated electronic pop-up book. Its keyboard is cut in half along a jagged, staircaselike diagonal that weaves its way between the middle keys. With the lid closed, both pieces lie flat in the case, but the right half sits above the left. As the lid opens the two pieces slide together, forming a seamless typing surface that stretches beyond the confines of the case, an inch on each side, like wings. Hence the computer’s nickname, the butterfly.

The expandable keyboard has gone from idea to product in less then two years. For IBM, not long ago considered a lumbering giant and an also-ran in the PC market, that represents real improvement. Consider the story of Track Point, the innovation that made IBM’s original ThinkPad series such a commercial success. The size and shape of a pencil eraser, the touch-sensitive button was a sleek replacement for the bulky mouse. Nevertheless, the concept languished for two years before being developed. IBM now recognizes that it has to do better. Karidis first showed a working model of his keyboard to Bruce Claflin, general manager of the personal-computer division, in April 1993. ““Do that again,’’ a delighted Claflin demanded. ““We made the decision in one minute,’’ Claflin recalls. Researchers and designers, many pulled off other projects, then devoted a year of intense effort to bring the 701C to market.

Can the butterfly catapult IBM from third in the portable-PC market – behind Toshiba and Compaq – to king of the heap? The machine has drawbacks such as a three-hour battery life (the best have at least six hours), a price of $3,800 for even the lowly no-frills model and an Intel 486 processor chip instead of the state-of-the-art Pentium that serves other deluxe model notebooks. IBM’s real problem, however, is not with the computer itself but with production. The company is already predicting shortages by this summer. If so,it would be the third time in a year thatBig Blue has failed to meet supply needs for a popular new product. IBM execs say they’ve just been blessed with unanticipatably high demand. True, maybe, but running out of product gives the competition time to rush cheap clones to the market. But IBM is betting that its design is subtle enough to thwart knockoffs for at least a year. It is a new sort of cockiness in the PC arena for the old computer maker, and for the first time in a long while it might even be justified.