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The strike by the union of Los Angeles teachers heads into its second day on Tuesday. In doing so, some 30,000 California-based teachers are exercising a right not enjoyed by teachers in most states.

California is among the minority of states that do permit teachers’ strikes even though most states allow collective bargaining and wage negotiations for public school teachers.

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According to the Center for Economic and Policy Research, as of January 2014, 35 states and the District of Columbia outlaw striking. Teacher strikes are legal in 12 states and not covered in statutes or case law in three.

In 1935, Congress passed the National Labor Relations Act (NLRA), which was designed to achieve uniform, effective enforcement of a national labor policy. Congress intentionally exempted states and cities from the NLRA, because government employees traditionally didn’t enjoy a right to strike.

Consequently, the protections of the NLRA apply to private employers, rather than public employers, and leaves states free to regulate labor relationships with their public employees. The result is a wide variation of rules that vary drastically from state to state.

As a default rule, public employees have neither the right to strike nor the right to collectively bargain, unless such rights are provided by a state statute.

In California, labor relations between most local public entities and their employees are governed by a state law called the Meyers-Milias-Brown Act (MMBA), which recognizes the collective bargaining rights of public employees. The MMBA imposes on local public entities a duty to meet and confer in good faith with representatives of employee organizations, in order to reach binding agreements governing wages, hours and working conditions of the agencies’ employees.

If negotiations break down, California allows public employees to go on strike to enforce their collective bargaining demands, unless the striking employees perform jobs that are essential to public welfare, like firefighters and law enforcement personnel. The state’s supreme court has long held that strikes by public employees are lawful, unless the strike creates a “substantial and imminent” threat to public health or safety.