1. What is the purpose of a zipper clause? Over the years, the company had unilaterally implemented several plant rules (e.g., use of company telephones, excused absences from work), including the following two rules:

1. Perfect attendance—All employees who have a perfect attendance record in a 12-month calendar year will receive special recognition from the Company. (Note: There was no evidence introduced to show that any employee had ever received any special recognition under the announced rule before the implementation of the new attendance bonus rule.)

2. Discharges—The company has the right to discharge or discipline any employee for just cause.

The term ‘‘just cause’’ shall include . . . drinking or possessing any alcoholic beverage on company property or company time or reporting for work while under the influence of alcohol or drugs.

During the term of the current labor agreement, the company announced the implementation of two new work rules:

1. As of March 1, any injuries requiring treatment will now be accompanied by a drug/alcohol test.

2. As of March 1, any employee who achieves perfect attendance for a week will receive a wage bonus of $1 per hour for that week. Any employee who achieves perfect attendance for an entire month will receive a total wage bonus of $1.50 per hour for the month.