The simplest structure of the backpack is the knapsack. The knapsack is just a simple main pocket. The fabric can be canvas, cotton or polyester. The shoulder strap is just a simple rope. The mouth of the knapsack is made into a string pocket. The knapsack can also be used as a bundle rope at the mouth of the knapsack as the switch of the knapsack. This kind of backpack can’t carry too heavy things. It’s generally only used as a yoga bag or training bag, carrying clothes for exercise.