Set of skateboard trucks. You choose truck size based on the size board you are riding. Decks under 7.5" ride 4.25-4.75" trucks. 7.5" decks use 4.75" or 5.0" (most common) trucks. 7.6-7.9" decks use 5.0" trucks. 8.0" decks use either 5.0" (most common) or 5.5" trucks. Over 8.0", Longboard and Oldschool decks use 6" or 7" trucks.