The most salient characteristic of the music from "Bluebeard's Castle" is the importance of the minor second, an interval whose dissonance is used repeatedly in both slow and fast passages to evoke aching sadness/disquiet or danger/shock respectively. The minor second is referred to as the 'blood' motif, for it is used whenever Judith notices blood in the castle. Overall the music is not atonal, although it is often polytonal, with more than one key center operating simultaneously (e.g. the leadup to the climactic opening of the fifth door). However, there are some passages (for example, door 3) where the music is tonal and mostly consonant. Many critics have found an overall key plan, as one would find in a tonal piece of music. The opera starts in a mode of F, modulating towards C in the middle of the piece (tonally, the greatest possible distance from F), before returning to F towards the end. The text and setting at these points has suggested to some that the F-C dichotomy represents darkness/light.