2. One Octave is the Popular Song Range
The popular song is introduced to the public by vaudeville performers, cabaret singers, and demonstrators, whose voices have not a wide range. Even some of the most successful vaudeville stars have not extraordinary voices. Usually the vaudeville performer cannot compass a range of much more than an octave. The cabaret singer who has command of more than seven notes is rare, and the demonstrator in the department store and the five-and ten-cent store usually has a voice little better than the person who purchases. Therefore the composer of a song is restricted to the range of one octave. Sometimes, it is true, a song is written in "one-one," or even "one-two" (one or two notes more than an octave), but even such "rangey" songs make use of these notes only in the verses and confine the chorus to a single octave. But in the end, the necessity for the composer's writing his song within one octave to make an effective offering for his introducing singers, works out to his advantage. The average voice of an octave range is that possessed by those who buy popular songs to sing at home.
Now here is a helpful hint and another bit of evidence from the music angle, to emphasize the necessity for the perfect fitting of words and music. Let me state it as Berlin did, in an article written for the Green Book Magazine:
3. Melodies Should Go Up on Open Vowels
"Melodies should go up on open vowels in the lyrics—A, I or O. E is half open and U is closed. Going up on a closed vowel makes enunciation difficult."
Experience is the only thing warranted to convince beyond doubt, so test this rule on your own piano. Then take down the most popular songs you have in your collection and measure them by it.
4. Put "Punch" in Music Wherever Possible
As we shall see later, another definition of the popular song-hit might be, "A song with a punch in the lyrics and a punch in the music." Berlin expressed the application to the problem of melody by the following:
"In the 'International Rag,' for example, I got my punch by means of my melody. I used the triplet, the freak, from out of my bag of tricks:
Raggedy melody,
Full of originality.