of travel to make a significant pitch difference, and the sharp corners on the prongs may cause the string to break where the string passes over them. Most players use a fine tuner on the E-string even if the other strings are not so equipped. The A string is tuned first to a reference pitch source such as a tuning fork or, in most orchestras, the oboe, typically to 440 Hz, although some orchestras tune to another standard A such as 442, or even as high as 445 or 446 Hz to produce a brighter sound. Early music groups interested in authentic performance may use a lower standard A. When playing with a fixed-pitch instrument such as a piano or accordion, the violinist must tune to accommodate that instrument. The other strings are then tuned to the A in intervals of perfect fifths by bowing them in pairs. This puts the open strings in just intonation, which means the lower strings may sound flat compared to their nominal equal-tempered equivalents. In practice, this means some compromises must be made, and the strings not always tuned in perfect fifths, particularly on the viola and cello, where the lowest string is the C, three fifths away from the reference A, making the difference more apparent. Small, temporary tuning adjustments can also be made by stretching a string with the hand. A string may be flattened by pulling it above the fingerboard, or sharpened by pressing the part of the string in the pegbox. These techniques may be useful in performance, reducing the ill effects of an out-of-tune string until the arrival of a rest or other opportunity to tune properly. Tuning the violin, especially with the pegs, can cause the bridge to lean, usually toward the fingerboard. If left that way, it will warp. After tuning, experienced players typically check that the bridge is standing straight and centered between the inner nicks of the f holes, since bridges are free to move about, being held in place only by friction and the tension of the strings. Capable violinists know how to straighten and center a bridge; this can easily be done under normal string tension without damage to the instrument. The tuning G-D-A-E is used for the great majority of all violin music. However, any number of other tunings are occasionally employed (for example, tuning the G string up to A), both