  Another recent development was the introduction in the early 1990s of structures called effect algebras  (Foulis and Bennett [1994]) generalizing the orthoalgebras discussed in sect 4.1.  The definition is almost identical, except that the weaker condition \(a \perp a \Rightarrow a = 0\) is replaced by the weaker condition \(a \perp 1 \ \Rightarrow \ a = 0\). Like orthoalgebras, effect algebras are partially ordered by setting \(a \leq b\) iff \(b = a \oplus c\) for some \(c \perp  a\).[20] 