A knapsack is a bag with two straps that you wear over your shoulders, leaving your arms free. It is also known as a backpack, rucksack, pack, booksack, bookbag, or backsack. The word knapsack was the usual name for a rucksack or backpack up until the middle of the 20th century, and it is still commonly used in Canada. The primary difference between a knapsack and a backpack is the size, with a knapsack being smaller, with a capacity under 30 liters. Knapsacks are handy for carrying small items, and they are often used as school bags or day bags.