Tension type headache (TTH) is the most common headache type. About 40% (40 out of 100) of people have had a TTH. They are most often less severe than migraine headaches. A small number of people will have very severe TTH. When they are very severe, or happen very often, they can be very hard to tell apart from migraine. We used to believe that these headaches were caused by stress or muscle tension (hence the name). It has become clear that muscle tension is associated with all kinds of headache, including migraine.