Combinator, LLC (YC) is an American technology startup accelerator and venture capital firm launched in March 2005[1] which has been used to launch more than 4,000 companies.[2] The accelerator program started in Boston and Mountain View, expanded to San Francisco in 2019, and was entirely online during the COVID-19 pandemic.[3] Companies started via Y Combinator include Airbnb, Coinbase, Cruise, DoorDash, Dropbox, Instacart, Reddit, Stripe, Lezzoo, and Twitch.[4]