Musical characteristics and lyrics
There are several music genres Nine Inch Nails has been described as, including alternative rock, industrial rock, industrial dance, industrial, electro-industrial, industrial metal, and electronic rock. AllMusic's Steve Huey states that "Nine Inch Nails were the most popular industrial group ever and were largely responsible for bringing the music to a mass audience". Reznor has never referred to his own work as industrial music, but admits to borrowing techniques from such early industrial bands as Throbbing Gristle and Test Dept. Despite the disparity between those artists initially operating under the term "industrial" and Nine Inch Nails, it has become common in journalistic descriptions of Reznor's body of work to describe it as such. Reznor acknowledged in Spin magazine that "Down in It" was influenced by early Skinny Puppy, particularly the band's song "Dig It"; other songs from Pretty Hate Machine and With Teeth have been described as synth-pop.:35 Reviewing The Fragile, critic Steve Cooper noted that the album juxtaposes widely varied genres, such as solo piano in "The Frail" and drum and bass elements in "Starfuckers, Inc." Ambient music has been featured in some of Nine Inch Nails' music, including on Ghosts I–IV (which is specifically dark ambient), Hesitation Marks, The Downward Spiral, The Slip, and The Fragile.
Certain techniques and styles can be found throughout Nine Inch Nails' catalog. Songs such as "Wish", and "The Day the World Went Away" exhibit terraced dynamics. Reznor's singing follows a similar pattern, frequently moving from whispers to screams. He also has used software to alter his voice in several songs, as evident in "Starfuckers, Inc." and "Burn". The band's music also occasionally contains complex time signatures, notably in "The Collector", from With Teeth, and concert favorite "March of the Pigs". Reznor regularly uses noise and distortion in his song arrangements, and incorporates dissonance with chromatic melody and/or harmony. These techniques are all used in the song "Hurt", which contains a highly dissonant tritone played on guitar during the verses, a B5#11, emphasized when Reznor sings the eleventh note on the word "I" every time the B/E# dyad is played. "Closer" concludes with a chromatic piano motif: The melody is debuted during the second verse of "Piggy" on organ, then reappears in power chords at drop D tuning throughout the chorus of "Heresy", while an inverted (ascending) version is used throughout "A Warm Place", and then recurs in its original state for the final time on "The Downward Spiral". On The Fragile, Reznor revisits this technique of repeating a motif multiple times throughout different songs, either on a different musical instrument, with a transposed harmony, or in an altered tempo. Many of the songs on Year Zero contain an extended instrumental ending, which encompasses the entire second half of the three-minute long "The Great Destroyer". Allmusic's review described the album's laptop-mixed sound: "guitars squall against glitches, beeps, pops, and blotches of blurry sonic attacks. Percussion looms large, distorted, organic, looped, screwed, spindled and broken."
Lyrical themes found in Nine Inch Nails songs are largely concerned with dark explorations of the self ranging from religion, greed, fame, lust, addiction, self-deception, aging, regret, nihilism, and occasionally politics, with the latter topic often being scrutinized in Year Zero. Three of Nine Inch Nails' recordings are concept albums: The Downward Spiral, follow-up The Fragile, and the aforementioned Year Zero. With Teeth (under the working title Bleedthrough) was to be a concept album about an endless dream occurring in reality, but Reznor eventually took this idea out of the record.