What are the parts of a fugue?

Good question. By "parts", I assume you mean either

1. The names of the individual parts.

2. The various sections and their characters

I will answer both. First though, a Fugue can be written for as many "voices" (Soprano, Alto, Tenor, Bass are the usual names, no matter what instrument is actually being played) as the composer wants. However, the number of voices typically stays the same throughout. In other words, while not every section uses every voice, it's unlikely that you will see a voice ADDED once the music in underway. In general, the fugues of Bach are mainly written for 3, 4, or 5 voices.

1. The Names of the Individual Parts

Fugues are more or less built up of one (sometimes two, called a "double fugue) main part, called the SUBJECT. The SUBJECT ideally lends itself toward all manner of polyphonic treatment or musical "gymnastics". For the beginning listener, the SUBJECT is usually the only thing that sticks out enough to be heard - there are many "entries" of the SUBJECT throughout the fugue, mostly signifying the beginning of another EPISODE (which I will talk about below).

After the SUBJECT has been played (or "entered"), there is usually an ANSWER directly after. The ANSWER sounds very similar to the SUBJECT, but a closer listen will reveal that there are some minor changes. Using the 2nd fugue (C minor) from Bach's Well-Tempered Clavichord as an example, the ANSWER appears in the top voice of Measure 3, beginning on the note G. Compare and contrast this melody with the opening theme (SUBJECT) of the piece. You will also notice in Measure 3 that the voice which previously played the original SUBJECT now plays a downward scale passage, followed by some jagged 8th notes which don't seem to "resolve" themselves until the 1st beat of Measure 5. This is the COUNTERSUBJECT, a part which fits perfectly against both the ANSWER and the SUBJECT. Whenever this shape appears, you can rest assured that you will find the shape of the SUBJECT in one of the accompanying parts. Look throughout this fugue and see how many times you can spot this shape. Observe what else is going on when you see this part being played.

Other parts can be found in more complicated fugues, but they will all center around the idea of a SUBJECT and its accompanying parts. There are too many variations to list here, but you will find all that covered in the books I listed below.


2. The Various "Sections" of the Fugue.

Typically, musical analysts will section off the fugue into three MAIN areas.
A. Exposition (Introduction of each voice)
B. Development(the main body of the piece)
C. Recapitulation (the summary and ending)
Those areas are broken into smaller bits by identifying how the materials are used. The development is usually broken down into EPISODES, where the material switches (sometimes not so clearly) from one "groove" or "texture" into something else. In the Bach C minor fugue (Well Tempered Clavichord, Book I, Fugue 2) Measure 9 is an example of the beginning of an EPISODE. You can see how the music sort of "finishes up" what it was doing before and how it begins to do something new - playing with the "subject" in the top two voices while the Bass part starts a nice long running line down the keyboard.

The important things to know is that things are not as clear-cut and objectively discernible as in, say, a RONDO or something like that. Analyzing the form of music relies on the presence of SOME subjectivity. The other thing is that FUGUE is less a form than a "style of writing" and as a result, you will usually find more "exceptions" than rules (odd, isn't it?).

Rather than recommending the "usual" books on fugue, I will endorse two here.

1."Fugue and Invention in Theory and Practice"
by John W. Verrall
2."Sonata Forms"
by Charles Rosen - officially not a study of fugue, but there are many links between sonata form and fugal treatments in music that this book has turned out to be very valuable in my own teaching.

I have provided a space on my own website (www.pianobody.com) where I hope to answer questions like this for people with other theory questions. In the meantime, I am always available for other questions about music theory, performance, etc.

Good luck figuring it all out!

I think they are mainly soprano, alto, bass. I'm not sure about the structure of fugues except it always returns to the tonic.