A double-chamber bong. The best type of bongs to use. Smoothest smoke, double filtration and made out of glass/pyrex.
A nice and relatively large shaped first chamber with a curved stem that enters the smaller second chamber containing an upright stem and the 'bowl'.
Both of the chambers stems sit submerged in the water. Thus, the smoke is filtered firstly by the smaller chamber and then again by the larger chamber before you breath in the smoke.