The temperature method is based on the fact that the body temperature increases by at least 0.5°C at ovulation (in response to progesterone, a hormone secreted at the time of ovulation). The body temperature remains at this higher level until the next menstrual bleed (14 days on average).
This method is based on the principle of identifying the rise in temperature by taking your temperature each morning (with a reliable thermometer before getting up) and recording it on a graph (which is available in pharmacies or from your doctor). In practice, unprotected sexual intercourse is "permitted" only for 24 hours before this phase in which the temperature rises.
Warning, the temperature may rise in the same way because of an infection, even one that is not serious, waking or getting the sufferer up at night, and nightwork.