Concepts and general guidelines:
Repetition of Notes
Notes are repeated in a sequential cycle, hence memorable.
Repetition of Rhythm
In a vein similar to repetition of notes, we now repeat the same rhythmic sequence, changing the notes.
However, if these techniques are overly used, it can become boring.
The Secret is "Change"..
It is important to have at least one juxtaposed section in the riff.
One common structure is AAAB.
One small change at the 4th bar can take away the boredom of repetition!
In general these are how notes feel in relation to the chord they are on:
(numbers denote the interval on the scale of the key of the song)
1 - Expected
2 - Unexpected
3 - Expected
4 - Unexpected
5 - Expected
6 - Unexpected
7 - Unexpected
Case Study for Riffs
Let's study a song riff that uses the above techniques strongly, Melee's - Built to Last.
Original key is in C#. For explanatory purposes we shall talk in C major.
C (bar 1 & 2) Am (bar 3 & 4)
Notes in the Riff: Notes in the Riff :
B C E D E G
7 1 3 4 5 b7 (due to minor)
F (bar 5) G (bar 6)
Notes in the Riff: Juxtaposed Section, pivot point, rhythm changes.
E F A
7 1 3
C (bar 7 & 8)
Notes in the Riff:
B C E
7 1 3
Let us look at the part before the juxtaposed section.
The rhythm of the phrase is repeated.
Notes change, but each phrase starts on an unexpected note, hence a similar emotion at the start on each phrase.
On the 6th bar, rhythm structure and notes change, to take the role of the juxtaposed section.
It is possible to bang on a repetitive rhythm with repetitive notes. Just take note of chord's tonal color.