[Zhang Luying]

There are two types of guide rails: the square (profile) guide rail and round guide rail. Round guide rails originated first but do not provide the precision offered by square guide rail components. However, the round guide rail is the preferred guide rail for vertical movements with heavy loads. The application will dictate which type of linear guide rails should be used.
While square linear guide rails once cost many times more than round technology due to the extensive grinding requirements inherent in the design, new manufacturing techniques and economies of scale have encouraged engineers to entertain the use of square guide rails in a broader application space. Profile linear guide rails can now be found in many of the same applications that were once only supported by round guide rail.
Square (profile) guide rails can outperform round guide rails in specific circumstances that demand higher rigidity and more compactness in terms of load capacity relative to size. In general, profile rail guide rails offer higher load capacity, accuracy and stiffness, and a longer life expectancy.

[dmalicky]

I thought that sounded familiar:
http://www.thomsonlinear.com/website...uide_rails.php

[islamCe]

useful article