data.KEYSIGNATURE Key signature may be indicated by a value showing where the key is in the circle of fifths. Mixed key signatures, e.g. those consisting of a mixture of flats and sharps, and key signatures with unorthodox placement of the accidentals (Read, p. 143) must be indicated by setting the key.sig attribute to 'mixed' and providing explicit key signature information in the key.sig.mixed attribute.
Module MEI
Used by
<rng:data type= "token">
<rng:param name= "pattern"> mixed|0|([1-9]|1[0-2])[f|s] </rng:param>

MEI Guidelines Version 3.0.0