The 'F' attachment or 'plug' is a rotary valve triggered by the players left thumb. This gives the trombone access to an additional piece of tubing, which extends the range down to the instrument's fundamental 'pedal' tone, and also makes it easier to reach 7Th position. Until the 1980s, the type of rotary valve employed was the same as that found in the French horn, but this created additional resistance in the trombone as the air was forced through tight angles when the valve was engaged. This altered the sound of the instrument, and players weren't too happy with the added effort required. The solution was the 'Thayer' valve, which is found on most modern trombones. This introduces very little extra resistance, and the instrument seems to be more 'free blowing' as a result.