变格定弦（意大利语：Scordatura，英语：cross-tuning）是一种运用于弦乐器的写作技术。作曲家要求弦乐演奏者将乐器定弦至与惯常不同的音高。从而拉奏出独奏的声调，尤其是在表演不协调和弦、泛音或以正常方式不能制造的和弦组合等。

变格定弦并非近代音乐才会采用的手法，早于巴洛克时代，巴赫、韦华第、比贝尔等人已经运用。尤其是用于维奥尔琴上，因为它拥有多条弦线，为了方便演奏及令演奏者用较容易的指法去弹奏，便会将个别的弦线高音作出调整。后来便推至提琴类的乐器，包括小提琴和大提琴等。

Scordatura [skordaˈtuːra] (literally Italian for "mistuning"), is a tuning of a stringed instrument different from the normal, standard tuning. It typically attempts to allow special effects or unusual chords or timbre, or to make certain passages easier to play. It is common to notate the finger position as if played in regular tuning, while the actual pitch resulting is altered (scordatura notation). When all the strings are tuned by the same interval up or down, as in the case of the viola in Mozart's Sinfonia Concertante for Violin, Viola and Orchestra, the part is transposed as a whole.