Cross tuning refers to any of a number of alternate tunings for the fiddle. Typically a violin is tuned in fifths, to the notes E, A, D, and G (from high to low). This arrangement is known as standard tuning, and is uniquely suited to the requirements of classical music (and most styles of western music) due to the logical and systematized fingerings that arise.
Alternative tunings for the violin vied with the now-standard system of fifths during the early history of the instrument, before many aspects of it's form had been codified. This experimentation extended throughout the Baroque period and well into the Classical, before finally giving way to the now almost universal standard tuning.
In traditional fiddle music and most country blues, however, alternative tunings continued to be used and flourish long after fading from common use in classical, jazz and pop. The alternative tunings became known collectively as "cross tuning" due to acoustic parallels between the strings that seem to connect them cross-ways. An alternate explanation of the term states that cross tunings allow for more cross-string bowing.
The most common cross tunings for the fiddle are cross-A, cross-G, G-DAD (or double-D), triple-D and Black Mountain Rag tuning. Cross-A and Cross-G both tune the top two strings in parallel with the bottom two strings, the pair being separated by an octave. Cross-A is arranged E, A, E, A from high to low (the D and G strings are raised one full step to form E and A) while cross-G is arranged D, G, D, G from high to low (the E and A strings are lowered one full step to form D and G). Double-D tuning simply lowers the E string one full step to a D, forming a second, or double, D. Triple-D is the most radically different from standard tuning, with three of the four strings of the instrument being tuned to three octaves of D (the A string remaining untouched). Triple D is an extremely archaic tuning.
Cross tuning dramatically increases the resonance, tone and volume of the instrument, while simultaneously limiting the flexibility of tune/song selection. The tunings are associated with a specific musical key, and before a piece can be played in a different key, the instrument must be retuned. Cross tunings were used long before amplification existed to enhance the performance capacities of instruments. In rural America, they often spontaneously arose during the auto-didactic process of solo musicianship since the absence of formal musical education forced the individual to invent his own tunings. Cross tunings allow for the addition of more double-stops, drone notes, and create sympathetic tones amongst the strings, and thus were the natural choice for geographically or culturally isolated fiddle players. Legendary North Carolina fiddler and National Heritage Arts Fellow Tommy Jarell, for example, had never been exposed to standard tuning prior to his "discovery" in the early 1970's by a new generation of traditional music revivalists.