Basses, also known as tubas, are very large brass musical instruments that provide a low bass sound. They are widely used in orchestras and concert bands as well as brass bands where they are always referred to “basses”.
The image above shows the usual form of basses/tubas as large instruments held in front of the player. They can be supported on a small stand when the player is seated, or by a shoulder strap when standing or marching. Other forms of tuba are used such as the Helicon tuba and Sousaphone, both of which are wrapped around the players body to make them easier to carry. For obvious reasons they are more popular in marching bands, jazz and street bands.
A traditional brass band has up 4 basses: 2 larger basses pitched in B♭ (“B flat”) and 2 smaller basses pitched in E♭ (E flat”). The B♭ basses are sometimes called BB♭ (“double B flat”) because they are pitched so low. Brass band basses are transposing instruments and the players read music written in treble clef but play notes 2 octaves below those written.
Fore more information see wikipedia.