Trance music is a subgenre of electronic dance music that developed in the 1990s. Perhaps the most ambiguous genre in the realm of electronic dance music (EDM), trance could be described as a melodic, more-or-less freeform style of music derived from a combination of techno and house. Regardless of its precise origins, to many club-goers, party-throwers, and EDM adherents, trance is held as a significant development within the greater sphere of (post-)modern dance music.
Elements of what would become trance music were being explored by industrial artists in the late 1980s. Most notably, Psychic TV's 1989 album Towards The Infinite Beat, featuring drawn out and monotonous patterns with short but repeating voice samples, is considered by some to be the first trance album, but this claim is widely contested. The intent was to make sound that was hypnotic to its listeners.These industrial artists were largely dissociated from rave culture, and their trance albums were generally experiments, not an attempt to start a new genre with an associated culture -- they remained firmly rooted culturally in industrial and avant-garde music. As trance became to take off in the rave culture, most of these artists abandoned the genre.
By the mid-1990s, trance, specifically progressive trance, had emerged commercially as one of the dominant genres of EDM. Progressive trance set in stone the basic formula of modern trance by becoming even more focused on the anthemic basslines and lead melodies, moving away from hypnotic, repetitive, arppegiated analog synth patterns and spacey pads. Popular elements and anthemic pads became more wide-spread, compositions leaned towards incremental changes (aka progressive structures), sometimes composed in thirds (like Brian Transeau frequently does), buildups and breakdowns became longer and more exaggerated. The sound became more and more excessive and overblown. This sound came to be known as anthem trance. Immensely popular, trance found itself less filling a niche as edgier than house, more soothing than drum-n-bass, and more melodic than techno. It became more accessible to more people. Artists like, Paul Van Dyk, Ferry Corsten, and Armin Van Buuren came to the forefront as premier producers and remixers, bringing with them the emotional, "epic" feel of the style. Meanwhile, DJs like Paul Oakenfold, DJ Tiesto, and DJ Jean were championing the sound in the clubs and through the sale of pre-recorded mixes. By the end of the 1990s, trance remained commercially huge but had fractured into an extremely diverse genre.
Contemporary trance culture is heavily intertwined with recreational drug use. At present, trance is as much about who plays the music as it is about what it sounds like. Trance has transcended the underground scene to become the most popular form of electronic dance music, and a figure in the realm of popular music. For more concrete examples, check out any number of purported trance compilations; perhaps the most highly recommendable source would be the Global Underground series, including its "Nubreed" sub-series, because it captures the diversity of the genre as expressed through many of its brightest DJ talents. Also recommended as source material would be the Tranceport/Perfecto Presents... series, any of Sasha & Digweed's Northern Exposure mixes, and any of the mixes in the Renaissance series. The Labels to reference would include 3Beat, Bedrock, Devolution, Fluid, Fragrant, Hooj Choons, Hook, Perfecto, Vandit, Armada, Positiva, Harthouse, Eye Q, MFS, Platipus, NOOM, R&S, Yoshi Toshi, and ATCR Trance Music.
Trance is a form of music best characterized by quarter note drum patterns, and 16th/32nd note rhythm synthesizer patterns. It has a meter of 4/4 always, with a quarter note bassdrum acting as metronome, and quarter note high-hat hits offset. This unwavering drum mechanism may be constantly tweaked with for effect, with the attack, decay, resonance, frequency, tone, delay, reverb all given liberal treatment. The tempo is generally around 130-150 beats per minute (bpm). The rhythm section consists mostly of a repeating 1-4-5 (A-D-E) 32nd note sequencing arpeggio, and a bass section of minor whole notes usually drifting through the aeolian scale (though not always).