in isolation lowers the pitch to A natural. But together, these two valves produce a sharp G natural, because the second valve slide, which is long enough to lengthen a B horn to A natural, is slightly too short to lower the longer A horn to G. Combinations using all three valves have even worse intonation, so instruments need some means to compensate. Trumpets and flugelhorns commonly have saddles or triggers on the first and/or third valve slides to allow lengthening of the valve slides, while on larger instruments like the euphonium and tuba, such devices are impractical. Instead, better-quality tubas and euphoniums are provided a fourth valve, which takes the place of the 1-3 valve combination and allows use of 2-4 in place of 1-2-3. Better manufacturers of three-valve instruments such as Sousaphones often make the third valve slide long enough to partially address this deficiency. Also, many tuba models place the first valve slide in a position where it may be manipulated by the musician with the left hand during playing, which allows tuning for the always-flat 5th partial harmonic at the C below middle C. Another solution for euphonium or tuba is called a compensating system, and these have been made in both three- and four-valve versions. In a compensating system, the tubing from the final valve is routed back to the first valve, and each of the valves before the final one is fitted with a short valve slide in addition to the normal ones. When the third or fourth valve, as applicable, is depressed in combination with any of the other valves, the additional tubing brings the pitch into tune. This has the primary effect of making the lowest octave playable including B natural (unplayable on non-compensating horns), and a secondary effect of putting low B natural and C natural into tune. Compensating systems add weight and "stuffiness" to a horn, so they are not commonly fitted to tubas. They find principal use in euphoniums, in which virtually all professional models are compensating. Some French horns have an altogether different system: they have a complete set of slides on each valve in B, parallel to the F slides. Pressing a separate trigger routes the air through the B slides vice the F slides, and via shorter tubing to the bell. Intonation is less of a problem in French horn, because it usually plays using higher