Two is the first even number and the only even prime number. The word comes from the Greek dyo and the Latin duo through the Old English twa. Early languages often had both feminine and masculine forms for two and so there are a lot of diverse roots related to "two-ness". Many "two" words use the Greek root bi; biannual, binary, biscuit, and biceps, for examples. Others come from the Old English twa, such as between, twilight, twist, and twin. From duo we get dual, duet, dubious (of two minds), duplex (two layers), and double. The Latin di gives us diploma (two papers) and dihedral. The earlier Greek dyo produces dyad, composed of two parts. Two is the only positive real number that gives the same result when added to itself as when multiplied by itself. It is conjectured that 2 is the only even integer that cannot be written as the sum of two primes (see Goldbach's conjecture) and it has recently been proved that 2 is the largest value of n for which the equation x n + y n = z n has nonzero integer solutions (see Fermat's last theorem). Two is the base of the binary number system.