A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection STI. The male condom is rolled onto an erect penis before intercourse and works by forming a physical barrier which blocks semen from entering the body of a sexual partner. Condoms as a method of preventing STIs have been used since at least The effectiveness of condoms, as of most forms of contraception , can be assessed two ways. Perfect use or method effectiveness rates only include people who use condoms properly and consistently. Actual use , or typical use effectiveness rates are of all condom users, including those who use condoms incorrectly or do not use condoms at every act of intercourse.