Franco was the first theorist to record the distinction between dual and triple time. The reader who is unacquainted with musical science should learn that the rhythms of music are like those of poetry. Instead of poetic feet, music has "measures" separated by vertical lines drawn through the staff, and called "bars." Measures are often called bars. The musical measure corresponds to the poetic foot. A bar with two beats in it is like a foot of two syllables, except that in music the accent is normally always on the first beat. A bar with three beats is like a dactyl, one accented and two unaccented syllables, or beats. Dual time, or measure, corresponds to a poetic rhythm made up of two-syllable feet; triple time to one of three-syllable feet. A polka is in dual time; a waltz, in triple time. Franco first explained these points, and insisted that triple ought to be used in church music for the naïve reason that its three beats in one bar made it resemble the perfection of the Holy Trinity, three persons in one God. He made many improvements in harmony, among others recognizing the third, already described, as a concord, though not a perfect one. Another important feature of Franco's teaching was his advocacy of contrary motion of parts. The manner of writing practised by Hucbald prescribed what is called parallel motion; that is, the melody of the _cantus firmus_ and that of the descant always rose or fell together. If the one ascended one interval the other did so, too. Contrary motion permits the parts to move in opposite directions, and this makes it possible to avoid such disagreeable arrangements as consecutive fourths or fifths, and so leads the way to a richer and more beautiful harmony. Others had already practised what Franco preached in regard to this matter, so the most significant part of his work was that which dealt with measure. In order to write the measured music it was necessary to have notes representing sounds of different duration, and these notes Franco either invented or adopted. Here are the four notes which he used:-- [Music: Longa. Brevis. Maxima or Duplex Longa. Semibrevis.]. These names mean "long," "short," "double long," and "half short." The short note had half the duration of the long, and the duplex longa double it, while the semibrevis was half the length of the short note. We still have notes called breve and semibreve.