If you could reduce a mobile phone to its essence, it would look like the Modu. This tiny phone, which is slightly larger than a domino, is capable of sending and receiving calls and text messages. It can store contacts and MP3s with up to 16 gigabytes of storage capacity, and it has a small but usable screen and a sparse keypad that lacks numbers. Launched this week at the Mobile World Congress in Barcelona, the Modu can be used as a stand-alone phone. But more important, it can also be slipped into a variety of "jackets," such as in-car MP3 players, Global Positioning Systems, and larger cell phones, that expand the Modu's functions and change its look.

Modu Mobile, the Israeli startup that launched the phone, is hoping to change the way that consumers think about their handhelds, explains Itay Sherman, the company's chief technology officer. Today, people generally have one phone that they use all the time, and they use it for a year or two because it's too expensive to buy a new model more frequently. But Sherman says that the idea of one phone for all occasions doesn't mesh with people's lifestyle. Sometimes you want to walk around with the smallest possible phone, he says; other times you want a good messaging device with a large keyboard, or a media player with a large screen. "Instead of buying a completely new phone, the jacket enables you to switch."

In making the Modu, Sherman says, there were a number of technical considerations. While semiconductor technology is at the point where chips are small enough to easily fit into the mini mobile, his team also had to shrink the phone's other features, such as the screen, keypad, and battery. The display, for instance, needed to be specially designed: it uses organic light-emitting diodes and is a mere one millimeter thick. (See "Super-Vivid, Super-Efficient Displays.") Knowing that it would be impractical to put a full, numbered keypad on the Modu, Sherman says, his team designed a simpler keypad that lets people access menus on the screen, similar to those of MP3 players. The lithium-ion polymer battery, which uses the same basic technology as traditional phone batteries, was customized to be thin and long, while still providing about 3 hours of talk time and 100 hours of standby.

Once a user plugs the Modu into a jacket, however, the features improve. "The jacket may also have a battery," says Sherman, and the combined device shares the load between the two batteries. "It extends the talk time and standby time."

One of the main innovations, says Sherman, is that the software that runs the Modu automatically reconfigures when it is put in another device. A resource file defines the way the Modu and jacket will work together. "Every jacket you plug into, you'll get a completely different experience, yet it keeps the basic functionality in all cases so that it's familiar to the user," he says.

Beyond cell-phone jackets, Modu Mobile will offer other consumer-electronics devices in which the phone module can be inserted, improving the basic functions of the device. For instance, a camera with the Modu could wirelessly send pictures to other phones, and a car entertainment system designed for the Modu could let a user access his MP3s while enabling hands-free calling.

This isn't the first time that consumer-electronics companies have tried to build modular phones, says Avi Greengart, the research director for mobile devices at Current Analysis, a market research firm. He points to IXI Mobile, the maker of the Ogo mobile messenger. "It had the notion of connecting multiple devices together via Bluetooth," he explains. A user would have a basic storage module and then connect to a large display or media player. However, the technology didn't catch on because few people think to buy a shell of a media player and then the other pieces to make it work, Greengart says.