Many modern tenor trombones include an extra attachment of tubing – about in length – which lowers the fundamental pitch from B to F. There are two different forms of this tubing, open wrap and traditional, or "closed", wrap. The traditional wrap is curved and fits inside the main tuning slide while the open wrap extends past the main tuning slide and only has one curve in it. The F attachment is engaged by using a "trigger" which operates a valve (this is different from the three-valved valve trombone). This type of trombone is typically built with a larger bore size () and is known as a B/F trombone, F-attachment trombone, or trigger trombone. Trombones without this feature are known as "straight" trombones.