This 2 by 4 is a piece of dimensional lumber used in construction in North America. The nominal size is 2 by 4 inches but the actual size is 1.5 by 3.5 in (38 by 89 mm)

In American folklore it is often used as a club to get someone's attention. During the anti-trust case, United States versus Microsoft, Judge Penfield Jackson gave this analogy to reporters for the New York Times.

He had a trained mule who could do all kinds of wonderful tricks. One day somebody asked him: "How do you do it? How do you train the mule to do all these amazing things?" "Well," he answered, "I'll show you."He took a 2-by-4 and whopped him upside the head.The mule was reeling and fell to his knees, and the trainer said: "You just have to get his attention."

U.S. v Microsoft, United States Court of Appeals District of Columbia, June 28, 2001 [1]

The photo by Michael Holley, June 2008.