When creating your own bass line, you have to know where you are going. You must know what the next note and chord will be before you play it, although instinct often makes this more of a subconscious decision. Besides knowing what's next in the short term, you also have to know what's going to happen overall in the song. This structure of a tune is also called the song form.

Song form is often associated with letters. Each new section of a song begins with a different letter in the alphabet. The only exception to this is musical introductions and endings. These are usually not lettered unless they appear in subsequent repetitions of the part. Most songs use repeats. Therefore, you're likely to see the letters reappearing later on and over and over.

The first section is called the A section. Unless a song repeats the same groove, chords, and melody over and over — which can be done effectively — then the next section is labeled the B section, then C, and so on. Some songs only have A and B sections. Having a D section or higher, although not unusual by any means, is less frequent. Unless you are analyzing classical or progressive rock, you are unlikely to venture too far into the alphabet. A typical pattern in any popular form of music is likely to show A and B sections repeated throughout with other letters appearing less frequently.

Another common way to look at musical structure or form, especially in modern commercial pop music, is to use terms such as introduction (intro for short), verse, chorus, bridge, solo, and coda (which might also be called an outro). When someone refers to modern commercial pop, that would usually include modern country hits, modern R & B, some hip-hop, plus modern rock. A tune in these genres will often start with some sort of intro then proceed to the verse, which is normally the section where lyrics are expressed most freely. At about the one-minute mark, give or take a few seconds, you'll usually hear the first chorus. The chorus is the section where the main emotional heft of the song is delivered with maximum impact. Here you often find the title of the song and a more repetitive vocal hook, as well as a lift in overall dynamics (volume). Usually the song then returns to the verse section but with different lyrics and perhaps some modifications or evolution of the arrangement and instrumentation.

After that, the second chorus section is likely to appear, and it's often identical to the first. The second chorus may be repeated twice in a row depending on the chorus's length. Next, usually a brand new section, the bridge, appears. The bridge often completes the song by providing the final emotional, lyrical, melodic, and harmonic content that the song heretofore has not tapped into. For instance, the lyric might offer up some additional, crucial information about the story being sung. Or the emotion of the piece might become more desperate. The bridge might also use an obvious chord that has been held at bay previously or saved just for the bridge. It is not uncommon to even venture into another key altogether — often the relative major or minor — only to find your way back to the original key after the bridge ends.

In most songs, tension and release are exploited for emotional effect. Further, after reaching the summit of the final chorus, you should feel like an interesting emotional tale was narrated through music. Often these final choruses will continue on, repeating over and over until some ending or outro can be devised or until the recording engineer fades the song out gradually.

Through your understanding of the structure of music, you can make better choices all around. First of all, you will know what section comes next. Second, you will be able to create contrasts or specific textures in your bass lines for each section. This is especially important when you are asked to write your own bass lines no matter what the genre.