6x6 lumber refers to wood that measures approximately 6 inches in both width and height. However, in reality, due to the milling and drying process, a nominal 6x6 lumber may actually measure slightly less, typically around 5.5 inches by 5.5 inches.

6x6s are often used in construction for applications that require strong and sturdy support, such as in the construction of decks, fences, retaining walls, and other structural elements. They are known for their durability and ability to bear heavy loads, making them a popular choice for various outdoor and load-bearing projects.