Seven Sisters Look up Seven Sisters at Dictionary.com
"the Pleiades," early 15c. (see Pleiades), seven daughters of Atlas and Pleione, placed among the stars by Zeus. As a name for the major multi-national petroleum companies, it is attested from 1962. They were listed in 1976 as Exxon, Mobil, Gulf, Standard Oil of California, Texaco, British Petroleum, and Royal Dutch Shell. The Pleiades also are known as the Seven Stars (Old English sibunsterri), though in 15c. and after this name occasionally was given to the Big Dipper (which also has seven stars).
FOOL: ... The reason why the
seven stars are no more than seven is a pretty reason.
KING LEAR: Because they are not eight?
FOOL: Yes, indeed: thou wouldst make a good fool.
["King Lear," Act I, Scene V]