A letter believed to be a love letter by Beethoven, known as the "Letter to the Immortal Beloved," sparked intense debate among later generations regarding its recipient. Beethoven is said to have written the love letter in 1826, while on a journey to a Bohemian spa resort.

Why would this modest work, incomparable to the gravitas of Beethoven's other trios, be connected to the "Immortal Beloved," who is presumed to be the object of his fervent affection? It is almost impossible not to suspect a connection. Among several women whose names have been suggested, the foremost candidate for the "beloved" is Antonie Brentano. While the dedication of WoO 39 was indeed to a woman, it was to Antonie's daughter, Maximiliane. Her name was not only inscribed on the autograph manuscript of WoO 39 along with the date June 26, 1812, but Beethoven also presented the score with the dedication: "For my little friend Maxe Brentano, to encourage her when she plays the piano." Beethoven's journey took place immediately after this.

 While Beethoven might give the impression of being difficult and formidable based on his famous portraits, his letters often reveal a kind-hearted and delicate side. The same applies to this work. The autograph piano part contains fingerings by the composer, which might suggest special consideration for the daughter of his beloved. The scarcity of extremely wide-ranging chords and leaps in the piano part might also indicate that he intended to make it easy for a 10-year-old girl with small hands to play.

The work is a single-movement piece adhering to the typical sonata form, and its short development section also features a simple construction with fragments of existing material. The scale and clarity of the piece also suggest that a form suitable for a child's performance was chosen. However, the coda beginning in the subdominant key is striking, and the fact that the scale of this coda surpasses that of the exposition, development, and recapitulation sections aligns with Beethoven's other ambitious works, in that it no longer adheres to the standards of late 18th-century sonata form. Furthermore, the coda features the most substantial thematic development in the piece, including melodic variations of the theme and motivic imitation, making it rich not only in scale but also in content.