Aggressive skates are identifiable from recreational or speed skates by a prominent gap in between the second and third wheels (The H-Block) which allows for grinds perpendicular to the direction of the wheels. A hard plastic surface on the sole of the boot known as a "Sole plate" or "Soul Plate" allows grinds parallel to the direction of the wheels. From these grind surfaces comes a lexicon of well known grind stances, though sliding can occur on any surface of the boot or wheels. Aggressive skates typically have much smaller wheels than a traditional inline skates. The small size allows for more freedom when grinding as there is less risk of catching on obstacles. Additionally these small wheels feature a flat profile to accommodate the impact from jumping tall heights. In recent years aggressive skates have begun to adopt larger frames and wheels, in what is seen as a bridging of the various inline disciplines.