Horizontal-branch stars play an important rôle in defining the absolute distance scale in the universe .
For that one has to know M _ { V } at relevant B - V with well defined [ M/H ] for each type of horizontal-branch star .
The nature of the horizontal-branch stars is reviewed with emphasis on all the open questions regarding evolution and structure , questions which show that these stars are not the easiest objects in the distance scale discussions .

Using the HB stars with the best Hipparcos parallaxes , the HB at ( B - V ) _ { 0 } = 0.20 and [ Fe/H ] = -1.5 has M _ { V } = +0.71 .
Assuming the [ Fe/H ] to M _ { V } relation for RR Lyrae stars to be valid for HB stars too , one finds M _ { V } \simeq 1.00 for [ Fe/H ] = 0 .