If predictions are correct, an incredibly rare Macdonald viola could fetch $44 million in an upcoming Sotheby’s auction. That’s nearly triple what the next most expensive instrument put to auction was: $16 million for, you guessed it: Another of Stradivari’s instruments, a violin was sold to benefit the victims of the 2011 earthquake and tsunami in Japan.

The Macdonald viola is considered a masterpiece of the undisputed master luthier Antonio Stradivari. He built stringed instruments from the mid-17th century until his death at the age of 93 in 1737. They are commonly held in museums, lent to the most talented performing musicians and bought by collectors. They're often sought out by thieves too.

Why so much for this particular instrument? Stradivari made around a thousand instruments by hand in his lifetime, and around 650 survive today. Most were violins. He made far fewer violas, which are slightly larger and have a lower register than a violin. Only 10 of Stradivari's violas exist today. 

Only it and one other, which is held at the Library of Congress in Washington, D.C., are is privately owned. So this is probably the only time you’ll see a Stradivarius viola up for sale.

It's also an almost perfect instrument. The Australian composer Alfred Hill told the Rudolph Wurlitzer Co. of New York in 1926: “I have seen every existing specimen (of Stradivari’s violas), and judged as a whole, I place this viola at the top; its proportions are untouched and, in fact, as left by the maker.”

It dates from Stradivari's "golden period" from 1700-1720, when Stradivari made perhaps his best instruments. And it's in almost perfect shape -- for a 300-year- old instrument, that is. Unlike many Stradivarius instruments, the Macdonald still has most of its varnish on it and has almost no nicks or cracks. 

David Aaron Carpenter, a viola master who is the only person allowed to play it between now and the auction, says it's like “Stradivari handed it to you from his workshop.”

A perfectly crafted, one-of-a-kind instrument from the most highly sought after luthier from his most prolific period, which will only be available once? There couldn't be a more desirable instrument for collectors, which is why some willing to shell out huge fortunes for the Macdonald.

Watch Carpenter play Isaac Albeniz's frantic 1892 composition, "Asturias" on the Macdonald, in what he says was the pinnacle of his career. "Every moment up until this point has prepared me to get to this moment and show the world what an instrument of this caliber can really do," Carpenter said.