To Guido d'Arezzo is ascribed its development up to some semblance of our present system, although the claim has often been denied. It is certain, however, that the innovations were made at this period. In the first place Guido made the red line _always_ stand for the pitch of F, and at a little distance above it he added another line, this time yellow, which was to indicate the pitch of C. Thus the signs began to take very definite meaning as regards pitch; for, given a sign extending from one line to the other, the reader could see at a glance that the music progressed a fifth, from F to C, or _vice-versa_. And now the copyists, seeing the value of these lines in determining the pitch of the different signs, of their own account added two more in black ink, one of which they drew between the F and the C line, and the other above the C line, thus [illustration]. By doing this they accurately decided the pitch of every note, for the lowest line, being F, the line between that and the C line must stand for A, and the two spaces for G and B; the top line would stand for E, and the space between it and the yellow line for D. Little by little these copyists grew careless about making the lines in yellow, red, and black, and sometimes drew them all in black or red, thereby losing the distinguishing mark of the F and C lines. In order to remedy this, Guido placed the letters F and C before the lines representing these notes, thus [illustration]. In this way our modern _clefs_ (_clavis_ or key) originated, for the C clef, as it is called, gradually changed its shape to [illustration] and [illustration], and the F clef changed to [illustration], which is our bass clef in a rudimentary form. Later, still another line was added to the set, thus giving us our modern staff, and another clef, [illustration], was added on the next to the lowest line. This, in turn, became our present treble clef, [G:]. In the course of time the signs themselves underwent many changes, until at last from [Podium], etc., they became our modern signs.