Trade was the mainstay of the urban economy in the Middle East， as caravans negotiated the surrounding desert， restricted only by access to water and by mountain ranges. [TPO 16-1-1]
the mainstay of something
A mainstay holds everything together， whether it’s your grandmother who acts as the anchor of your whole crazy family， a pillar that physically keeps a building from falling down， or the job that enables you to pay your rent every month. The noun mainstay is originally a nautical term meaning the rope that stabilizes two masts on a sailboat， and since the 1780s it’s been used to mean “chief support.”
a） an important part of something that makes it possible for it to work properly or continue to exist
Agriculture is still the mainstay of the country’s economy.
Cocoa is the country’s economic mainstay.
Even during conflict， I argue that family and friends are the mainstays of this world.
Salon（Nov 25， 2016）
b） someone who does most of the important work for a group or organization：
She was the mainstay of the team.
He has been a mainstay at baseball and basketball games.
New York Times（Nov 14， 2016）
Trump countered on election day， piling up huge majorities among the non-college-educated， blue-collar whites who have long been the mainstay of his support.
Los Angeles Times（Nov 09， 2016）