Research in Motion Ltd., the maker of BlackBerry phones, is set to reveal today a phone that folds in half, a departure from the slab-like design that has defined its products.

The long-rumored phone will be called the BlackBerry Pearl Flip and will be available from T-Mobile USA and with overseas carriers later this year, at an undisclosed price.

The “flip” or “clamshell” design, where the display and keyboard are separated by a hinge, is a popular one for conventional cell phones, particularly in the United States. Jim Balsillie, co-chief executive of RIM, said 70 percent of handsets in the country have this shape.

“Bringing this form factor to the smart-phone category is, we think, very special,” Balsillie said. “Smart” phones expand the features of a regular phone with applications such as Web browsing and e-mail access, which is RIM’s forte.

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