The Duo for Violin and Piano, originally titled Rondo for Violin and Piano because of its structure, was written in 1947 and premièred in 1948 by violinist Eugene Kash and John Newmark at the piano. Morawetz later changed the name to Duo because: "to my great surprise not only laymen but even professional musicians seem to think that the word RONDO implies a work of a happy nature and that certainly does not fit the character of this composition."

Since that time it has become the most performed Canadian compositions. It has been recorded three times on disc by the Canadian Broadcasting Corporation (by Albert Pratz, Martha Hidy and Joseph Pach) and was in the repertoire of many other well-known Canadian violinists including Elie Spivak, Hyman Goodman and Betty-Jean Hagen. It has also been played many times in the U. S. A. and it was introduced to audiences in Czechoslovakia and Russia by the brilliant Czech violinist Nora Grumlikova. Many young Canadian violinists have chosen this work for violin competitions or for their graduation recitals.

The work follows the form of classical rondos: A,B,A1,C,A2,Bl,A3. But Morawetz carefully avoids the rather monotonous repeat of the A section. At each appearance it is restated with new textures and in different colours. At the opening, the A section is only four bars long; it is in D minor, the only thing it has in common with subsequent appearances. It is stated by the violin and after a fast interlude we hear it again leading directly into the B section. In its next statement (A2) we hear the main theme in the piano, with a short extension in the violin.

The following C section is the longest. The violin starts with a fast melody in triplets which gradually changes into an expressive melody in quarter and half notes while the piano maintains the triplet motion underneath by playing fragments of the A theme. The last part of the C section consists of a long crescendo which leads into the dynamic climax of the piece, a powerful augmented statement of the opening theme in the piano (A3); the violin sustains the tension by a nervous repetition of the note D in an octave doublestop until both instruments resolve into a fortissimo G sharp minor triad.

The following Bl section resembles the B statement but it ends very softly and leads into the last appearance of the opening bars (A3). It is played in hollow octaves by the piano accompanied by mysterious muted tremolos in the violin. The music almost dies away in a Phrygian D minor cadence. But the violin suddenly brings back the fast original tempo which after a few bars brings the work to an abrupt fortissimo ending.