Time signatures are the area of western musical notation that give specifics about time and how it will function within the song. The importance of these two numbers is apparent by their placement alone; time signatures are literally the second thing a musician sees, after the clef. One glance at the time signatures and the musician is able to put the entire song into context; not only do time signatures denote rhythm, they can also tell quite a bit about style and genre.Time signatures consist of two numbers placed vertically on the staff. They appear right after the key signature (if there is one) or the clef (if there isn't). Each number says something different about the piece of music: the top note indicates how many beats are in one measure of music, the bottom indicates which type of note makes a beat (or "gets the beat," in musician-speak). For example, times signatures of 4/4 tell a musician that there are four beats per measure of music and that the quarter note is one beat.
But time signatures aren't always so easy. The above explanation only applies to one type of time signatures, simple time signatures. 4/4 is the most common of the simple time signatures, but 3/4 and 2/4 are used frequently, as well. The other type of time signatures, compound time signatures, tends to be a bit more complicated. Compound time signatures are recognizable by a top number of six or higher that is always divisible by three, like 6/8. To figure out how many beats exist per measure, the top number is divided by three; so in 6/8 time signatures, the top number wouldn't say that there are six beats per measure, but rather two. Likewise, the note type is found by multiplying the bottom number's note value by three; an eighth note doesn't get the beat in 6/8 times signatures, but the dotted quarter note does (a quick note: the note value in compound time signatures will always be dotted). 6/8 and 12/8 are two of the most common compound time signatures, used heavily in waltzes and blues, respectively.
But even compound time signatures don't cover the broad spectrum of rhythms that time signatures have to offer. No, there are still times that a rhythm is so odd, so seemingly offbeat, that the time signatures must be similarly irregular. And that's how we get to irregular time signatures, time signatures that can be categorized into simple or compound, but because of their rare usage and strange rhythm aren't really categorized as such -- like 5/8 or 10/4. Time signatures like these are found pretty frequently in innovative rock and pop groups. The most famous examples of irregular times signatures are found all over Pink Floyd's "Dark Side of the Moon," but bands like Rush and Genesis (early Genesis, before Phil Collins stepped away from the drums) have also used them to marvelous effect.
