If the beard is light enough that the skin can be seen, then what is underneath it must be washed. However, if the beard is thick, then everything that covers the skin from the beard must be washed: that is, washing all the hair that grows on the cheeks, including the sideburns and the chin. Washing what is underneath is not required because the obligation is to wash the face, and when hair grows, what is underneath it is no longer considered part of the face; therefore, it does not need to be washed. The same applies to washing under the mustache and the eyebrows. See: Al-Durr Al-Mukhtar 1: 69.