Something strange occurs in math, doing: \placeformula\startformula H(K|M,C) = H(K|C) - H(M|C)\eqno{\hbox{(\in{}[eq:keyapp])}} \stopformula produces the following error: ------------------------------------ ! You can't use `\eqno' in math mode. <argument> \normalreqno {\doformulanumber [][][]{}} \secondoftwoarguments #1#2->#2 \dostopformula ->\doplaceformulanumber \getvalue {\e!stop \formulaparameter ... l.20 \stopformula ------------------------------------ Why cannot \eqno be used in math mode? It is its raison d'etre! Plain TeX typesets the following input without problems: $$ a = b + c\eqno{(12)} $$ \end Strange enough the following does work as expected: \placeformula\startformula \eqalignno{ H(K|M,C) &= H(K|C) - H(M|C)&\eqno{\hbox{(\in{}[eq:keyapp])}}\cr } \stopformula Is this a little bug perhaps? Hans van der Meer