Rhythm is the different durations of tones (sound). The tones may be changing in pitch, but each tone has a duration. These time durations are symbolized by the shape of a note.
To begin counting rhythm, we should know the different note shapes. Two different notes are a quarter note and an eighth note. The quarter note is a full beat in a measure. The eighth note is a half of a beat, or a half beat.
Here are four measures of mixed quarter notes and eighth notes. Each measure has four beats, counted 1 2 3 4. Since eighth notes cannot make a whole beat, the rest of the duration of a beat is counted & (and). So we can count a measure as:
The eighth notes are tied together by a beam (connecting them, in place of each flag). Notice there is no '&' after a quarter note. The quarter note is still sounding until the next downbeat. Mentally, though, you can think of its total beat as '1 &', or '2&', etc. Keeping a constant pace in your head by mentally counting '1&2&3&4&' is a good idea. This helps to predict where, in time, the beat falls.
Add a total half beat (eighth note) to another total half beat, and it equals a whole beat (quarter note). Don't stop counting the whole beat until the beginning of the next beat, because we need both of the half beats when we say '1&'.
Another good idea is tapping your foot, because the downbeat falls when your foot touches the ground, and the upbeat is when your foot is all the way up. Personally, I pulse my toes because it's quicker and quieter. Do whatever works to help you keep the beat within the tempo (even duration of all the measures).
Rhythm may be interpreted in different ways, which is exciting. The above picture of four measures, may be divided into logical musical phrases:
One interpretation of this rhythm, is that the first four quarter notes are an introduction (four solid downbeats mark out the rhythm and tempo). These beats may crescendo to open up the next measure. After the intro, the second measure seems to begin the song, with a strong first count.
Next, two identical phrases echo each other. Their two eighths introduce two strong quarter note downbeats. Play the eighths softer than the two quarters for more effect. Notice that repetition, coupled with the downbeats in different parts of the measures, creates suspense.
The fourth measure has two eighths for the first beat. This begins a new rhythmic phrase that grew out of the last one. The new phrase is four eighths and a quarter, repeated. After the second four eighth notes, there's no downbeat yet, but we may assume it's going to be a quarter note that ends the phrase.
You may interpret the rhythm in a different way, which is perfect. Employ whatever way helps to keep track of the beat and tempo. Learning is easier with a collection of reference points.
Finally, here's my interpretive breakdown of the four measures:
1 2 3 4 1 / 2& 3 4 1& 2 3 / 4&1& 2 3&4& (1)
//// / --//--// ----/----(/)
Together, these phrases may feel like an incomplete rhythm pattern, since only three phrases appear, waiting for the fourth to make a bar. It's interesting that even with our strong downbeats, the music can seem incomplete, until we satisfy our sense of timing with another rhythm, based on a bar of 4 in this instance.
Notice that measure barlines, | |, demark an equal duration of measured time. However, notes travel along this musical 'sidewalk' in meaningful phrase groups, unaware of barlines. This sense of rhythm is beyond written measures.
Just as melodies are phrases of notes that relay ideas to us, rhythm supports the total idea of a melody or song. We break up phrases into measured segments, to reproduce a song. Both note phrases and rhythm work together. Spiritually speaking, music is never just symbols on a page, but inspiration.
Recommended reading: Sound Waves
The next page explains time signatures, that combine the number of counts per measure with different note types per count.