NEXT TIME you sing "Happy Birthday to You," look out. You might have to pay royalties.

In fact, if you own one of those Casio digital watches that play the song every hour on your birthday, you already have paid an extra penny for the privilege.

Every year or so, some TV special or sitcom gets a stiff little letter from the Summy-Birchard Co. of Princeton, N.J., bringing the somewhat startling message that the ditty--next to "For He's a Jolly Good Fellow" and "Auld Lang Syne" the most-sung song in the language--is copyrighted, and will be until the year 2010.

"It's true," said Ernest Farmer, an S-B executive. "It's used all the time on TV. On the stage, too, and the movies. People often don't even notice, it's so familiar."

It was in "10," as you probably don't recall. Also "The Great Santini," "Oh God!" and "Fame." When "The Gin Game" with Cronyn and Tandy came to Broadway, somebody in the opening night audience noticed the song wasn't credited on the program. Summy-Birchard sent one of its famous letters, and the producers paid $25 a performance thereafter.

The annual income from the song approaches six figures.

It goes to the Hill Foundation of Chicago for children's education and also, needless to say, is a steady source of good news for Summy-Birchard. The song was written by two sisters, Patty Smith Hill (lyrics) and Mildred Hill (melody), kindergarten teachers in Louisville in the 1890s. Patty, who got a doctorate in education at Columbia and later taught at that university, was an innovator in preschool learning. She and her sister believed in songs as an educational tool and published a Sunday school book of songs in 1893.

One of those early songs was something called "Good Morning to All," to be sung by teachers to their children. It went, "Good morning to you, good morning to you, good morning dear children, good morning to all." And the tune? You got it. The tune was that immortal melody.

The sisters tinkered with it over the years. Surely most Americans used to sing "Good Morning Dear Teacher" in grade school. That was a good one, but the greatest variation was yet to come.

"Happy Birthday" burst upon the American scene in 1935.

(It is uncertain who wrote the version that goes, "Happy birthday to you, you belong in the zoo, you look like a monkey, and you act like one too." Surely not the Hill girls.)

Summy-Birchard came into the picture as the inheritors of Clayton Summy, a Chicago piano teacher who published the sisters' work. In the '30s, the firm joined the American Society of Composers and Publishers, which monitors the songs played on radio and TV and other public media. One of the first big hurdles was the suit against Western Union, which used the song in singing telegrams. That suit was dropped, but the principle was established, and today most transgressors come around after they get the letter. People seem to get a kick out of the whole idea.

There are other surprising copyrights: "When Irish Eyes Are Smiling," for instance. A lot of school hymns are private property. Paul McCartney owns "On Wisconsin" for reasons not immediately apparent.

Mildred Hill died in 1916 and Patty in 1946, but their song goes on . . . and the money rolls in.