The Romantic period in Danish classical music is intimately linked to the idea of a particular Nordic sound; this has its roots in the efforts to establish a national musical theatre in Denmark in the late 18th century. Elements which later came to be associated with the “Nordic sound”  were present in the work of Johan Ernst Hartmann (1726–1793), especially his compositions Balder’s død (“The death of Balder”, 1779), drawing on Nordic mythology, and Fiskerne (“The fishermen”, 1780).

In the early 19th century, composers such as Christoph Ernst Friedrich Weyse (1774–1842) and Daniel Friedrich Rudolph Kuhlau (1786–1832) developed the Nordic sound, drawing on folk songs, medieval songs, and ballads. A relatively simple folk-song-inspired tone combined with a soft slow rhythm and a melody in a minor key came to constitute the Nordic sound, although the notion of a Nordic sound can hardly be defined in clear musicological terms.

Simultaneously, the popularity of the musical theatre increased, and musical elements such as songs, dances, and orchestral pieces were increasingly introduced in plays, most notably the great celebrational plays that were performed in honour of the monarchy. One such celebrational play, written on the occasion of a royal wedding, became a huge success: Elverhøj (“Hill of elves”, 1828), written by Johan Ludvig Heiberg (1791–1860) and with music by Kuhlau.

In the following years, folk song came to be seen in increasingly Romantic terms, as part of the Danish national heritage. Andreas Peter Berggreen’s (1801–1880) Folk Sange og Melodier (“Folks-songs and melodies”, 10 vols, 1842-71) bolstered this attitude; gradually, the appreciation of folk song as Danish cultural heritage shifted from an aesthetic to a national-ideological mode, focusing on its place in the Danish national musical tradition, which centred around the Nordic sound.

In 1840, the young violinist and composer Niels Wilhelm Gade (1817–1890) composed Gjenklang af Ossian (“Echo of Ossian”), which was published by the major Leipzig-based publisher Breitkopf & Härtel and performed in Leipzig in 1842. The following year, Mendelssohn performed Gade’s First Symphony for the first time. It appears from the response to his compositions that his success was facilitated, in particular, by what the German audience perceived as his Nordic sound.

In 1847, Mendelssohn died and Gade, after finishing the season in Leipzig, returned to Denmark on the eve of the Slesvig-Holstein War of 1848-50. He aspired to do in Copenhagen what Mendelssohn had done in Leipzig and to use his experience to develop a musical environment for the Copenhagen middle  classes. In terms of composition, his primary task was to turn the dark tone with which his “Nordic pieces” had become associated into something more nationally Danish. This resulted in the cantata Elverskud (“Spellbound by elves”, 1853), based on the text of two medieval Danish folk songs and incorporating characteristic folk-song intonations.

The development of the Nordic tone into an artistic folk tone is also associated with Johan Peter Emilius Hartmann (grandson of the above-mentioned namesake, and known as the “great Hartmann”; 1805–1900), in particular his compositions to August Bournonville’s ballets and his folk-song opera Liden Kirsten (“The humble Kirsten”, 1844-66). Hartmann evinced his nationally-Danish inspiration by writing music to  the plays or poems of Adam Oehlenschläger: the melodrama Guldhornerne (“The golden horns”, 1832), the overture Axel og Valborg (1856), and incidental music to Hakon Jarl  (“Earl Hakon”, 1844-57).

A Nordic tone is also present in the opera Drot og Marsk (1878, “King and Marshal”), composed by Peter Heise (1830–1879) in the aftermath of the Danish defeat in the Second Slesvig-Holstein War of 1864. Most notably, he employed it in the romances of the only commoner in the opera, the innocent Aase. Heise wished to emphasize her popular characteristics and deliberately drew on Danish medieval folk songs for the purpose.

The artistic folk-song tone continued to influence Danish classical music until  the end of the century. However, it was no longer unchallenged. While it remained the dominant Romantic – and to a growing extent conservative – mode for composers in the tradition of Gade and Hartmann, composers with other ideals sought to extricate the folk songs (which were still a major source of inspiration) from their Romantic treatment, and to rediscover their authenticity. Within church music and secular folk-school music, Carl Nielsen (1865–1931) and Thomas Laub (1852–1927) played significant roles in this movement, which would become dominant in Danish classical music after 1900. Nielsen also occasionally drew on national landscape evocations for his orchestral work, e.g. his rhapsodic overture En fantasirejse til Færøerne (“An imaginary voyage to the Faroes”, 1927).