The distance between the trucks, measured from the board's inside mounting holes. This distance will determine the "feel" of the board. A shorter wheelbase will feel as if it turns quicker when you need to whip the board around, but will feel less stable when you are going fast. Longer wheelbases feel more stable, but are slower when trying to turn. Most boards now have about a 14 inch wheelbase. Most oldschool boards had about a 15 - 15.5 inch wheelbase. Old freestyle boards had very short maybe as short as 12 inch wheelbases.