Movement I - Measures 9 to 12

In measures 9-12, Beethoven adds a fifth voice at the end of each phrase.

The easiest way to score this is to have the viola play the bottom two voices in measure ten (allowing it to use the open C string for the double stop), which means giving the cello the middle voice (F to E). When the phrase repeats an octave lower, the cello can take the double stops. Thus a literal transcription would look like this:

Beethoven's actual arrangement is as follows:

In measure nine, Beethoven leaves out the alto voice. This allows him to end the preceding phrase with the staccato chord in the second violin and viola as discussed in the previous post. But it also accentuates the contrast between this measure and the following measure. We now move from a two-voice texture to a five-voice texture. He further accentuates this contrast both by adding a crescendo in measure ten (dropping back down to piano when we revert to the two-voice texture) and by moving the entrance of the fifth voice from the third beat of measure ten to the first beat. (Although practical considerations might have something to do with this change as well. I imagine the viola part is easier to play this way.)

At the beginning of measure 11, he adds an A in the second violin part that is not present in the original. This makes for better chord spacing. Probably the only reason he did not have this A in the original was that in was not possible on the piano (at least not if he wanted half notes in the alto part).

In the last measure, he drops the fifth voice altogether, keeping the low C but eliminating the chromatic line B-B flat-A.  Presumably Beethoven thought retaining the line in this register would sound muddy, especially when the lower two voices resolved to a third at the beginning of measure 13.