New York (CNN Business) Nearly 50,000 General Motors (GM) employees across the United States walked out late Sunday night after GM and the United Auto Workers union failed to reach an agreement on a new four-year contract.

Workers participating in the strike are spread across 31 General Motors factories and 21 other facilities in nine states.

Here's what you may have missed:

Why this strike is a big deal

This is the US auto industry's first strike in 12 years, and it's also the biggest strike by any US labor organization in the same time span. GM, America's largest automaker, has nearly 50,000 full-time and temporary UAW members on its payroll. The last autoworkers strike was also against GM, and lasted three days.

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