What is the difference between Romantic and 20th century music?

There is often an overlap between Romantic and 20th century music, so they are easily confused. My advice is to listen to as many different composers as you can from both these periods until you learn how to reliably distinguish between the two. Here are some predominant features that you can use to identify in each period:Romantic music- rich, chromatic harmonies; increased use of dissonance e.g. suspensions that don't always resolve; modulations into unrelated keys, ambiguous tonality/atonal/modal, cross-rhythms, use of extremes e.g. range of an instrument and dynamics; unusual and non-traditional structure (modified sonata form), use of expressive intervals in the melodic line, rubato (especially in piano pieces), nationalistic features (Grieg used folk dances), irregular phrasing, sudden shifts in mood/colour etc.20th century music- extreme use of dissonance (especially semitones), listen to the style is it jazzy, 12 tone, pentatonic, minimalist etc.? Atonal or lack of a stable tonality, non-classical time signatures e.g. 7/4; almost impossible to count rhythms (have a look at a score if you can), unusual grouping of musicians (listen to 'Gruppen' by Stockhausen), use of electronics, synthesisers (Listen to Steve Reich's 'Different Trains'), use of non-classical instruments (often a mixing of different cultures in a piece), no sense of form or structure, exploits new techniques on instruments e.g. flutter-tonguing, extreme contrasts in dynamics etc.N.B. not every feature will be applicable to every piece, but several will apply so that you can clearly distinguish.