Mendelssohn's engagement with chamber music can be broadly divided into two periods. The early period extends until 1829, and the later period from 1837 onwards; both piano trios (Op. 49 and Op. 66) were composed in the latter period. Like the first trio, Op. 66 is in a minor key and consists of four movements. While the individual movement types adhere to classical multi-movement forms, the formal structures of each movement do not strictly conform to 18th-century archetypes, instead exhibiting a freer construction, as was common among many 19th-century composers.

In this work, the arpeggiated chord at the beginning of the first movement serves as a core motif for other themes within the movement and for themes in subsequent movements. One is encouraged to look for the intervals of the arpeggiated chord heard at the opening of the work, and for inverted forms of the do-mi-sol basic shape, in other themes (paying particular attention to the sixths appearing in melodies). Doing so will reveal Mendelssohn's masterful skill in motivic manipulation.

First Movement: Allegro energico e con fuoco

It begins with the persistent repetition of an arpeggiated chord, evoking the struggle of a suppressed state, as if confined to one place. Although in sonata form, there is no repetition of the exposition, and the proportion of the recapitulation and subsequent sections to the entire movement is remarkably large. This is because, after the recapitulation of the secondary theme, the development section is partially recapitulated, making the entire coda extensive (if the section from the recapitulation of the development onwards to the end of the movement is considered the coda, its measure count exceeds a quarter of the entire movement). If the somewhat formalized framework of sonata form as we currently conceive it differs from Mendelssohn's formal conception, then the section to be recapitulated might not necessarily extend only to the end of the secondary theme group. The diversity of sonata form has been repeatedly discussed, but the structure of this first movement prompts us to reconsider the conventional divisions of exposition, development, and recapitulation.

The tonic key of C minor comprises multiple themes (in such cases, referred to as a "primary theme group" rather than a "primary theme"); after the non-melodic, turbulent arpeggiated chord theme, the violin finally presents a melodic theme. Following another return of the opening theme, the music transitions to the relative major, E-flat major, where the strings play a flowing and bright secondary theme characteristic of Mendelssohn.

After a brief transition, the development section begins with imitation between the piano and violin based on the movement's opening theme. The writing style of the development section is also integrated into the recapitulation. For instance, in the recapitulation of the movement's opening theme, contrary to the exposition, while the strings first present the arpeggiated motif, the piano plays triplet arpeggios very similar to those that appeared with the secondary theme's melody in the development section. Then, as the primary theme's arpeggiated chord shifts from the strings to the piano, the violin, in turn, imitates the piano, creating a texture corresponding to the beginning of the development section. One could say that the recapitulation integrates the content of both the exposition and the development. Previously introduced material is still utilized in fresh ways in the coda. Here, note also how the strings superimpose the same theme in augmented note values over the piano's arpeggiated opening theme of the movement.

Second Movement: Andante espressivo

Ternary form. Each section is dominated by a distinct rhythm, so strongly characterized that they appear to be differentiated by it. That is, the first section (designated A) features a tranquil, syncopation-centric theme played homophonically. The middle section (designated B), beginning in E-flat minor, is woven around two melodies: the first, appearing in the cello, is primarily syncopated, creating a connection with section A, while the melody spun in the piano's high register is accompanied by a uniform rhythmic chordal accompaniment, and this eighth-note pulse continues throughout the section. Then, as if liberated from this rhythm, the piano shifts to flowing sixteenth notes, and the string material also becomes the thematic melody of section A, entering a modulating section, while the accompanying rhythm is maintained, leading to the recapitulation of section A. In the coda, which is about a quarter of the length of the entire movement, previously introduced material appears one after another. Furthermore, after a partial recapitulation of the music from section B, a climax is formed through the piling up of motives, a crescendo building to fortissimo, and chromaticism, resulting in substantial content that could be considered an independent section. The heightened music immediately diminishes in dynamics, and finally, the note values of the motives are extended, bringing the movement to a quiet close.

Third Movement: Scherzo Molto allegro quasi presto

This is a scherzo broadly divided into five sections, ABABA; if applied to traditional schematics, section A can be considered the main part, and section B in G major the trio. However, the correspondence between the sections is weak, and their structures differ significantly from each other.

It is a perpetuum mobile movement without a general pause from beginning to end, creating the so-called Mendelssohnian "elfin scherzo" through the strings' quick, light movements, generally subdued in dynamics (the Scherzo from A Midsummer Night's Dream is a representative example). In terms of figuration and melodic assignment, the two string instruments are treated as a pair for most of the movement, but the piano, moving independently from the strings, does not stand out. Rather, large, sweeping movements from the piano are avoided, and it blends with the rustling string instruments.

Focusing on the formal aspects, each section is further divided into several smaller parts, and individual parts have a simple structure where the same figuration is periodically repeated. However, in contrast to the clear and periodic divisions within each part, the boundaries between the larger sections are indistinct, giving the impression that one section has seamlessly transitioned into the next. One reason for this is likely that, within the continuous flow of the perpetuum mobile music, subtle changes are introduced to persistently repeated motives, or new motives are added at the end of repeated phrases, thus transitioning by gradually modifying the preceding music. When the beginning of section A returns, imitative writing also contributes to blurring the divisions.

Fourth Movement: Allegro appassionato

Particularly noteworthy in this movement is the appearance of a chorale-like melody, which is thought to be based on one of the following: "Gelobet seist du Jesu Christ," "Herr Gott dich alle loben wir," or "Vor deinen Thron tret ich hiermit." Mendelssohn's interest in J. S. Bach is well-known, and his experience with Bach's works may be related to the use of this melody.

When considering the form, the use of the movement's opening thematic melody or fragments thereof in the transitional sections connecting the main sections gives the impression of a rondo form. However, at the same time, while the second theme appears in E-flat major and the chorale theme in A-flat major in the first half, in the second half they appear in the movement's tonic C minor and its parallel major C major, respectively, which aligns with the treatment of secondary themes in sonata form (sometimes referred to as "free sonata form"). Yet, if pressed, the section corresponding to the development consists only of brief imitative passages based on the thematic melody, and there is no complete recapitulation of the theme in the tonic key, raising the question of whether it makes sense to force it into existing archetypes.

The striking primary theme in 6/8 time, reminiscent of a dance, appears not only in the aforementioned transitional sections but also as a counter-melody to other themes, lending unity to the entire movement. Meanwhile, regarding the thematic melody that first appears in E-flat major, its arpeggiated figuration has been noted for its commonality with the theme of the first movement.

Not only is the motivic transformation rich, but the changes in sonority are also abundant, ranging widely from thin, chamber-music-like textures to dense textures employing piano octaves and string double stops. The most powerful sound in the movement is likely the chorale at the end. Here, the three instruments cover a wide range, and the chorale is played majestically using tremolos and double stops spanning three to four strings. The appearance of a chorale-like theme in the major key with a solemn character at the end, after a continuous minor-key passage, is a development also seen in Mendelssohn's Preludes and Fugues Op. 35.