A collective agreement is an agreement in writing between an employer and the trade union representing employees of the employer which contains provisions respecting conditions of employment, rates of pay, hours of work, and the rights and obligations of the parties to the agreement. Ordinarily the agreement is for a definite period such as one, two or three years, but not less than twelve months. Under some conditions, amendments are made to agreements by mutual consent during the term of the agreement in order to deal with special circumstances.