Strange Tuning

Mozart's famous Sinfonia Concertante for violin and viola makes its effect not least through the unusual tuning of the strings of one of the solo instruments. Mozart asks the viola player to retune the strings half a tone higher than is usual. A process known by musicians a "scordatura". But what is the reason and what is the story behind this method of tuning instruments? Tom Service explains why "scordatura" is so significant and so effective.