Why did Beethoven use triplets for the whole first movement of Moonlight Sonata rather than writing it in say 6/8?

Question

I just came across the score for Beethoven's Moonlight Sonata, and I was surprised when I saw the time signature of the first movement.

(My background is that I've played music for many years—mainly guitars and keyboard—but not studied music theory beyond what was covered in piano lessons as a child. So I have a basic understanding of time signatures, note values, triplets and suchlike, and am used to working with them. Although while the C-shaped time signature symbol means 4/4 or Common Time, I did have to search to learn that the vertical line through it means either tempus imperfectum diminutum or Alla breve.)

My surprise came when I saw that the movement is written in 4/4 (or is it 2/2?), with the melody as triplets across it. Without really thinking about it, I had assumed it would be in something like 6/8.

Previously, I've only encountered triplets as occasional exceptions that bring a certain emphasis to short sections by their contrast. Yet here, triplets are the norm for the entire movement. What is going on here?

Answer 1

In piano music (at least) it fairly common to write in 4/4 time with all triplets. This most frequently happens when the melody itself is best expressed in 4/4, while the accompaniment, in triplets, could otherwise be expressed in 12/8.

Examples

"Moonlight" Sonata"
This is the case of the "Moonlight" Sonata. While the accompaniment is in triplets throughout, the melody is a "4/4" (actually 2/2) melody. The below image shows the initial entrance of the melody in measured 5-7.

(IMAGE SOURCE)

Expressing this in 12/8 would be somewhat misleading, in that it would imply the primacy of the triplets as well as being harder to read. (It would also suggest a 4-beat metric accent pattern rather than 2-beat.)

X: 1
T: Moonlight Sonata
T: in 12/8 time
M: 12/8
K: C# minor
L: 1/8
%%score (V1 | V2)V:V1
V:V2
[V:V1] z6 z3 (2:3:2G3/2G/2 | G6-G3 (2:3:2G3/2G/2 | G6
[V:V2 stem=down] G,CE G,CE G,CE G,CE | G,DF G,DF G,DF G,DF |

Chopin Nocturne in F Major (Op. 15, No. 1)

This kind of "12/8 written as 4/4" is characteristic of Chopin's Nocturnes. The accompaniment is often triplet based, but the melodies are 4/4. The following example, from Op. 15, No. 1 in F major, is written in 3/4, though the accompaniment is in 9/8.

(IMAGE SOURCE)

Chopin Noturne in Eb Major (Op. 9, No. 2)

By contrast, consider Chopin's Nocturne in Eb Major (Op. 9, No. 2). Here, the piece is given in 12/8; but note that in this case, the melody is also triplet based.

(IMAGE SOURCE)

Chopin Nocturne in B Major (Op. 9, No. 3)

A particularly illuminating example comes from Chopin's Nocturne in B major (Op. 9, No. 3), which employs both 6/8 (for the A section) and 2/2 (for the B section).

(IMAGE SOURCE)

Answer 2

If you look further in the score, you will notice sixteenth notes in the melody (dotted rhythm, strictly speaking). Notating or reading this in 6/8 would be much more awkward.

Compare clarity of the rhythm notation in these two measures:

See some more related discussion:

How to play this rhythm in Beethoven's Moonlight Sonata? as a 4/3 polyrythm or as 6/16?

Piano: quaver triplets in RH v dotted quaver and semiquaver in LH

Developing Hand independence and Finger independence

Why is the last semiquaver outside the last triplet of the measure?