From the squelchy riff throughout Luke Vibert's Analord to the screaming lead of Phuture's Acid Tracks, acid lines are a popular element of many dance songs, even spawning the whole genre of acid house. Most are played on the Roland TB-303 bassline synthesiser or one of its clones, although they are seldom basslines. So what exactly are they?

There doesn't seem to be a clear definition of an acid line, but they tend to share two common characteristics: firstly, they use repetitive notation; and secondly, they employ a filter that has a moving cutoff point.

Acid lines were first played on synthesisers which had step sequencers, and due to the technical constraints of the era in which they were created, the first acid lines tended to be up to sixteen notes or rests in length. Although the technology used to create electronic music has since improved, acid lines often still retain this characteristic simply because it is part of a style that has become very popular.

The other main characteristic of the acid line, the filter, is almost always of the low pass variety. The resulting sound depends on the synthesiser used and the exact type of filter that it has, but in general the effect is of the timbre getting duller as the cutoff point is lowered, and brighter as it is raised. If the filter's resonance is turned up and the synthesiser is overdriven, the effect can be accentuated to the point where it sounds less like an instrument increasing in brightness and more like some kind of twisted machine that's in increasing levels of distress.

The following songs have notable acid lines:

• A Guy Called Gerald: Voodoo Ray This is a rather laid back song considering that you can dance to it. Appropriately, the acid line is gentle and relatively slow, proving that acid lines don't have to be monstrous screams.
• Hardfloor: Acperience At roughly nine minutes long, this piece of music is an epic journey. Several acid lines are played simultaneously, although one in particular is in the foreground of the mix. The cutoff point is increased very slowly to give a feeling of rising tension, while the different time signatures employed by the different acid lines give you enough to hold your attention and the persistent four-to-the-floor house drumbeat ensures that you can dance to it. The music as a whole is much greater than the sum of its parts, and almost draws you into a trance-like state.
• Madonna: Ray of Light The acid line in this song is relegated to background duty, making the overall mix sound more exciting. It's worth noting that this acid line is merely a single note being constantly repeated, showing the extreme extent to which the timbre is more important than the melody.
• Fatboy Slim: Praise You Much like Ray of Light, this track has an acid line in the background to give the mix a bit more energy. Towards the end of the song it erupts into a noisy flurry, but otherwise it remains subdued, merely providing decoration to an already complete sounding mix.
• Josh Wink: Higher State of Consciousness Probably the most famous acid line of all is the one that fuels Josh Wink's phenomenally popular acid house track Higher State of Consciousness. It slowly builds up like a long crescendo, the cutoff point rising further and further to continually create more and more tension, until it finally reaches a screaming climax of noise.
• Luke Vibert: I Love Acid Not only does this song have a fun, bouncy acid line playing throughout, but the lyric itself is about Luke Vibert's love of the genre.

For more examples of acid lines, you can hunt down pretty much any release by Hardfloor, Plastikman, or Luke Vibert. All three have released countless tracks that exemplify the sound.