Tubes form the basis of relaxed cycling: If the tube fits perfectly on the rim and, when optimally inflated, forms a unit with the tire, it almost rolls by itself. Of course, the elastic tube can also be stretched a bit to fit a larger rim or compressed to adapt to a smaller rim - but both options can negatively affect the riding behavior and the lifespan of the material. Therefore, it's important to buy exactly the type that the rim requires, including the correct valve for the existing hole.