A yellow dog contract is an illegal contract that some employers will force upon employees before hiring them, where the employee agrees not to join any trade union while in the employment of that company.
Any yellow dog contract is illegal and can be financially compensated if a person takes a case against their employer for the same. Employers who do not want to follow laws relating to minimum wage and fair working acts do not want their place of work to have the involvement of any employment union because then they would be likely to be shut down by the union or forced to change those aspects of their work environment or job duty.