Chess is said to have developed in this period, where its early form in the 6th century was known as "chaturanga", which translates as "four divisions the military" – infantry, cavalry, elephantry, and chariotry – represented by the pieces that would evolve into the modern pawn, knight, bishop, and rook, respectively. Doctors also invented several medical instruments, and even performed operations. The Indian numerals which were the first positional base 10 numeral systems in the world originated from Gupta India. The names of the seven days in a week appeared at the start of the Gupta period based on Hindu deities and planets corresponding to the Roman names. The ancient Gupta text Kama Sutra by the Indian scholar Vatsyayana is widely considered to be the standard work on human sexual behaviour in Sanskrit literature.