Ski Boots are measured in Mondo Point sizing which is a measurement in centimeters. Many manufacturers produce shells and liners that are stamped with a range from the whole to the half size, i.e. 27.0 - 27.5. Inside the liner is a stock footbed or insole. That insole may be available in a thick and thin version to create the illusion of a "whole" size. The stock insoles do not offer much arch support and are recommended by both the boot manufacturers and Skis.com to be replaced with an aftermarket pre-formed or custom insole to give you better support.
Since the custom fit adjustments are much better than they used to be, many manufacturers have decided that producing both half and whole sizes isn't critical anymore. For this reason many of the ski boots available on Skis.com will only be available in sizes ending with .5. The refinement in the left navigation will show the sizes in ranges such as 27.0 - 27.5, and will return results for with sizes 27.0 and 27.5. For information on aftermarket footbeds or insoles to make the whole and half size boots fit the same please consult the Buying Guide for Footbeds and Insoles.