When Enrique, an aspiring heroin dealer from Nayarit, Mexico, arrived at his uncles’ home in California in 1989, they led him to a closet full of brand-new Levi’s 501 jeans.

“Take what you want,” the uncles, who were deep into the heroin trade, told Enrique.

That scene, described by Sam Quinones in his excellent book Dreamland, about America’s opioid crisis, marks a strange and important feature of the U.S. heroin market.

Mexican dealers, who peddle much of the black-tar heroin in the United States, valued Levi’s 501 jeans almost as much as currency. In fact, they sometimes accepted payment in denim form: One balloon could be had for two pairs. When the dealers came home to their ranchos wearing 501s—or if they brought them as gifts—their families knew they had truly made it.

The story behind why the pants were so valued explains much about foreign heroin dealers’ motivation for their work.

Quinones describes how 501s were the “gold standard” for men in the rural parts of Mexico in the 1990s. The pants were expensive there and hard to obtain. Owning a pair implied not only wealth, but likely American connections.

Soon, dealers’ families began to expect gifts of 501s upon their return to Mexico, so the dealers couldn’t not go back to sell more heroin, Quinones writes.