Unusually for a classical work, the nickname "Intimate Letters" was given by the composer, as it was inspired by his long and spiritual friendship with Kamila Stösslová, a married woman 38 years his junior. The composition was intended to reflect the character of their relationship as revealed in more than 700 letters they exchanged with each other.

The viola assumes a prominent role throughout the composition, as this instrument is intended to personify Kamila. The viola part was originally written for a viola d'amore, however the conventional viola was substituted when Janáček found the viola d'amore did not match the texture. Milan Škampa of the Smetana Quartet has interpreted the third "letter", or movement, as a lullaby for the son that Janáček and Kamila Stösslová never had together.

The work is essentially tonal albeit not in the traditional sense. For example, the work closes with six D-flat major chords (Janáček's favourite chord), but with the added dissonance of an E-flat.