The Under Mount slides give you the maximum width provided in the drawer space. With under mount drawers, the drawer slide is attached to the bottom of the drawer as opposed to the side. Higher-quality cabinet lines use under mount drawer slides over side-mount slides.
With under mounts drawer slides, you actually gain more interior space due to not having to compensate for the space of the slides on the side they are also slightly stronger than side mounts slides due to the weight being contained underneath. The sides of the drawer will remain bare because the metal slides will not be exposed... To find out more, see our page on self close drawers.