Brass instruments are generally divided into two types based on their 'bore' (the shape of the tubing): cylindrical or conical. Trumpets and trombones have a mostly cylindrical bore, which gives them a bright, directional sound. The tuba, however, has a conical bore, meaning the tubing gradually gets wider from the mouthpiece all the way to the bell. This conical shape is what gives the tuba its warm, mellow, and 'round' sound that blends so well with the rest of the orchestra. Because of this shape, the tuba is essentially a giant version of the cornet or the flugelhorn. The wide bore also makes it easier for the tuba to produce extremely low notes (the 'pedal tones') with a high degree of stability. This physics-based design choice is why the tuba is the foundational 'anchor' of the brass section.