The Stradivarius line of violins is widely believed to be of the finest quality - structurally and acoustically. Modern instrument manufacturers have long striven to replicate the violin's timbre, yet all - to date - have failed. The violins attract millions of dollars at auction and are considered some of the most valuable artifacts in existence. But what makes these instruments so desirable, and why have today's manufacturers failed to emulate their quality? In the 1600s, families often built their fortunes around a specific trade. For the Stradivari family of Cremona, Italy, violins and other stringed instruments were their speciality. As a teenager, Antonio Stradivari served as an apprentice in the workshop of respected violin maker Niccolo Amati. Stradivari went on to set up his own business and quickly developed a reputation as a master of his trade. Stradivari set about improving on the models produced by his mentor. The arches were altered, the colour of the varnish was tweaked and the thickness of the wood was toyed with until Stradivari designed a model that satisfied his standards. Stradivarius violins are a complex blend of spruce and maple wood for the interior, strip and neck, coupled with treatments of gum and minerals including potassium borate or borax. Theories abound as to why the instruments have such unique acoustic properties. Famed violinist Nicolo Paganini (1782-1840) claimed Stradivarius used only 'the wood of trees on which nightingales sang'. More recently, scientists have speculated that a dip in winter temperatures from 1645 to 1715 slowed tree growth, producing the ideal timber for violin makers. Others believe the wood was soaked in water, came from ancient buildings or had a special varnish that affected the tonal quality. Despite modern technology, the techniques Stradivari used are yet to be fully understood or replicated. There are many violins in circulation that bear a label featuring the Stradivari name, but that doesn't mean they were all created lovingly by the man himself. Many were made in the 19th century and modelled on the master's handiwork. The label wasn't meant to deceive a buyer, but to indicate the model on which an instrument was based. There are believed to be 650 genuine Stradivarius instruments in existence. Some are housed in humidity-controlled glass cases in museums while others, such as the Lady Tennant (made in 1699), on loan through an anonymous collector to Chinese violin virtuoso Yang Liu, continue to be enjoyed in concert halls around the world. While all Stradivarius violins are considered to be of exceptional quality, those produced in the early 1700s are the most sought after. On May 16, the so-called Hammer Stradivarius (1707), named after Swedish collector Christian Hammer, sold at auction for more than US$3.5 million. It is believed prices at private sales have exceeded this amount. Stradivari didn't make only violins; he also produced violas, cellos, mandolins and even a harp. His legacy lived on through his sons, who kept the family business going after his death.