The PowerBook 2400c is the closest Apple came to producing a sub-notebook. Although it was technically not a Duo, it had everything that was trademarked to the Duo: the slim, lightweight design, at a time when most Mac laptops were nearing the 8 pound mark. (The 2400c came in at only 4.4 pounds.)
When Apple introduced this machine in May 1997, it was intended at first only for the Mac market in Japan (and later expanded to include only Japan and the USA. Consequently, warranty services are only valid for these two nations.
Lighter than a PowerBook Duo 2300c and as fast as a standard PowerBook 3400, the 2400c packs an active-matrix 800x600 pixel screen. Its full array of ports was remarkable, but it still used an external disk drive. (It had no CD-ROM drive, however.)
The 2400c, however, has a very small keyboard, being originally targeted only for the Japanese Mac market.
Taiwan has a 2400c fan club for users of this increasingly rare laptop.