The lunate epsilon, , is not to be confused with the set membership symbol ; nor should the Latin uppercase epsilon, , be confused with the Greek uppercase (sigma). The symbol formula_3, first used in set theory and logic by Giuseppe Peano and now used in mathematics in general for set membership ("belongs to") evolved from the letter epsilon, since the symbol was originally used as an abbreviation for the Latin word "est". In addition, mathematicians often read the symbol as "element of", as in "1 is an element of the natural numbers" for formula_4, for example. As late as 1960, ε itself was used for set membership, while its negation "does not belong to" (now ) was denoted by (epsilon prime). Only gradually did a fully separate, stylized symbol take the place of epsilon in this role. In a related context, Peano also introduced the use of a backwards epsilon, , for the phrase "such that", although the abbreviation "s.t." is occasionally used in place of ϶ in informal cardinals.