Free A Player's Guide to Chords and Harmony by Jim Aikin

familiar with the basics, so a brief detour is in order.
     

MEASURES & BEATS
    Music is a time-based art. In order to structure time in a way that will allow music to be played, most forms of music divide time into more or less regular beats. The beat is the pulse of the music - and indeed, the beat will often be at about the same rate as a human pulse, or the rate of footsteps in either walking or running.
    Beats are grouped into larger units called bars or measures. The terms "bar" and "measure" are synonymous, with one exception: The vertical line between measures in a piece of sheet music is always called a bar line, never a measure line.
    The most common groupings place either three or four beats in each measure. For musicians, these groupings become almost second nature: When you're first learning an instrument, you may have to count "one-two-three-four" in order to learn the rhythm, emphasizing the first beat of each measure as a way of orienting yourself, but by the time you've been playing for a few years, you should be able to count the beats within a measure without ever thinking consciously about it. (As an aside, counting multi-measure rests seems to be difficult for some musicians. I'm not sure why this should be the case, but I suspect it's because they're relying on their intuitive understanding of beats rather than firing up the rational, mathematical part of the brain.)
    Other groupings of beats are sometimes heard. Measures consisting of five or seven beats each are difficult for some musicians to play, but their lively, exciting sound makes it worth the effort to learn how to count and play them. In some music, the number of beats per measure changes from time to time. This compositional technique is especially common in post-19th-century classical music. In pop music, the number of beats per measure will usually remain remain the same throughout a given piece.
    Most often, the rhythmic unit that lasts for one beat is the quarter-note. Other note values, such as the eighth-note, half-note, and dotted quarter-note, are sometimes used as the basic unit of the beat, but these are not often used except in classical music.

    The grouping of beats into measures is called the meter. The word "meter" comes, in fact, from a Greek root that means "measure:" The meter of a piece of music is indicated by its time signature, which is shown on sheet music as two numbers, one above the other, at the left end of the first staff. The bottom number indicates which note value (quarter-note, eighth-note, etc.) is used for the beat, and the top number shows how many beats are in each measure. Even if these ideas were entirely new to you, from the information above you'd quickly be able to figure out that the most commonly used time signatures are 4/4 (four quarter-notes in each measure) and 3/4 (three quarter-notes in each measure). Some other useful time signatures are shown in Figure 4-1.
    Time signatures are generally shown in text with a slash mark (/). However, they're not fractions.

    Figure 4-1. A few of the meters commonly used in popular and classical music. In 6/8, 12/8, and 5/8 time, the time signature indicates that the eighth-note is the rhythmic unit that receives the beat. But in meters that have more than four beats to the bar, the units that receive the beat are generally grouped into units of two or three beats each. In effect, the measure is subdivided into a smaller number of chunks. 6/8 is generally subdivided into two units, each a dotted quarter-note (three eighth-notes) in length. In 12/8, there are four dotted quarter subdivisions. In 5/8 or 5/4 meter, the subdivisions are of unequal length: A bar with five beats is felt as consisting of either three beats followed by two, or two beats followed by three.

     

ASSIGNING CHORDS TO MEASURES & BEATS
    In many pieces of music, when the chord that's being used changes, it always does so at the beginning of a new measure - at the bar line. This is more or