What is the name given to the pledge that workers in the past had to sign to agree not to join a union?

The term "yellow dog contracts" refers specifically to agreements that employers historically required workers to sign, in which the workers pled not to join or organize a union as a condition of employment. This practice was prevalent in the early 20th century, particularly during a time when labor unions were viewed unfavorably by many employers. The purpose of these contracts was to prevent workers from engaging in collective bargaining and to maintain control over labor relations. This practice has since been made illegal in many jurisdictions, reflecting changes in labor relations and workers' rights.

The other options do not accurately describe this particular agreement: "blue collar agreements" generally refer to various types of contracts related to wage and labor conditions but do not specifically address union membership. "No-union accords" imply agreements where unions are not allowed, but they are not specifically the historical contracts referenced in the question. "Employment assurance contracts" does not relate to the refusal of union membership and is more about assurances given to employees regarding their roles or job security.