A ‘tab’ is the built-in section of some Vitrazza Glass Chair Mat designs that bumps out to fit into the opening of a traditional desk style (generally 24-30" wide with drawer pedestals on each side). A tab is intended to keep the chair wheels on the mat if/when a user works very close to the desk. Most customers find they don’t work that close to the desk and therefore will not roll off the front of a mat that doesn’t have a tab. Also, if your desk has an open area between two legs that would enable a square or rectangle Glass Chair Mat to be positioned partly underneath the front of the desk, the extra fabrication for the notched-out tab is not necessary – you can get the effect of a tab just by sliding the square/rectangle partly under the desk.