Analyses of 17th-century stringed instruments suggest that a young Antonio Stradivari might have apprenticed with a particular craftsman.History is revealed in tree rings. They have been used to determine the ages of historical buildings as well as when Vikings first arrived in the Americas. Now, tree rings have shed light on a longstanding mystery in the rarefied world of multimillion-dollar musical instruments.By analyzing the wood of two 17th-century stringed instruments, a team of researchers has uncovered evidence of how Antonio Stradivari might have honed his craft, developing the skills used in the creation of the rare, namesake Stradivarius violins.Mauro Bernabei, a dendrochronologist at the Italian National Research Council in San Michele all’Adige, and his colleagues published their results last month in the journal Dendrochronologia, and their findings are consistent with the young Stradivari apprenticing with Nicola Amati, a master luthier roughly 40 years his senior. Such a link between the two acclaimed craftsmen has long been hypothesized.In the 17th and early 18th centuries, Stradivari created stringed instruments renowned for their craftsmanship and superior sound. “Stradivari is generally regarded as the best violin maker who ever lived,” said Kevin Kelly, a violin maker in Boston who has handled dozens of Stradivarius instruments.Only about 600 of Stradivari’s masterpieces survive today, all prized by collectors and performers alike. A Stradivarius violin currently on the auction block — the first such sale in decades — is predicted to fetch up to $20 million.An 18th-century depiction of Antonio Stradivari, the Italian crafter of instruments.World History Archive/AlamyStradivari likely learned his craft by apprenticing with an older mentor, as was customary at the time. That could have been Amati, who, by the mid-17th century, was well established and also living in Cremona, a city in what is now Italy.“Some people assume that because Stradivari was Cremonese and he was such a great violin maker, he must have apprenticed with Amati,” said Mr. Kelly, who was not involved in the new study.But evidence of a link between Stradivari and Amati has remained stubbornly tenuous: One violin made by Stradivari bears a label reading “Antonius Stradiuarius Cremonensis Alumnus Nicolaij Amati, Faciebat Anno 1666.” That wording implies that Stradivari was a pupil of Amati, said Mr. Kelly, but it was the only label like it that has surfaced.With the goal of shedding on this musical mystery, Dr. Bernabei and his team visited the Museum of the Conservatory of San Pietro a Majella in Naples and analyzed the wood of a small harp made by Stradivari in 1681. Using a digital camera, the researchers precisely measured the widths of 157 tree rings visible on the instrument’s spruce soundboard.A small harp by Stradivari from 1681.DeAgostini/Getty ImagesThe pattern created by plotting the width of tree rings, one after the other, is like a fingerprint. This is because the amount that a trees grows each year depends on the weather, water conditions and a slew of other factors, Dr. Bernabei said. “Plants record very accurately what happens in their surroundings.”The researchers compared their measurements from the Stradivari harp with other tree ring sequences measured from stringed instruments. Out of more than 600 records, one stood out for being astonishingly similar: a spruce soundboard from a cello made by Nicola Amati in 1679. “All the maximum and minimum values are coincident,” Dr. Bernabei said. “It’s like somebody split a trunk in two different parts.”The same wood was indeed used to make the Stradivari harp and the Amati cello, Dr. Bernabei and his colleagues suggest. This was consistent with the two craftsmen sharing a workshop, with the elder Amati possibly mentoring the younger Stradivari, the team concluded.Perhaps that is true, said Mr. Kelly, but it is not the only possibility. Instead, Mr. Amati and Stradivari might simply have purchased wood from the same person, he said. After all, luthiers in 17th-and 18th-century Cremona belonged to a small community, said Mr. Kelly. “They basically all lived on the same street.” More