As we all known, energy band is very important to everyone who want to go longer in physics, especially in condensed matter physics. However, as a student of physics, I am shamed to say that I cannot interpret the picture of energy bands well, thus would someone be kind to tell me the secret or refer me to some books or articles. As an example of my problems, I would take the picture attached(electronic structure of magnesium dibrode)as an example: how can one assign some curves belong to a band(in the picture, one could assign one of the curves to sigma bands which belongs to B-B)? And further more, if the curve stands for the energy of electrons in solids, what can we learn for the band structure? Since one coordinate of the picture is K vector, the change of energy wiht respect to K means what exactly? Thanks very much!