In music, a breve is the longest form of a note, originally considered the whole note by the 20th century it had been replaced as the whole note by the semi-breve, which is half it's duration. The breve was formerly represented by a square, but is now represented by an oval shape, with a line perpendicular to the stave on each of its sides. Research Breve
In music, C is the keynote of the normal or natural scale, which has neither flats nor sharps in its signature. It is also, the third note of the relative minor scale of the same. C after the clef is the mark of common time, in which each measure is a semi-breve.
In music, a crotchet is a time note, with a stem, having one quarter the value of a semi-breve, one half that of a minim, and twice that of a quaver. Research Crotchet