Heavier (thicker) strings need to be stretched tighter to produce the same pitch. The values in kg (kilogram) show why thicker strings are more difficult to play and bend, and also give a good idea of why a guitar should be adjusted according to the type of strings. If you have bought a guitar that came with a 009-042 string set and want to switch to, for example, a 011-049 set for a fuller sound, be sure to have your guitar adjusted to this tension: the curvature of the guitar neck must be adjusted, in many cases also the bridge, In some cases also the action (height between fret and string) and the comb. Once your guitar has been adjusted by a guitar specialist or luthier, it is advisable to place the same thickness and type of strings on your guitar afterwards.