Stems are audio files which consist of individual instrument tracks, isolated from the rest of the band, and usually (but not always) sourced from the official recordings. For example, you might find a set of stems for a metal song, where you have one audio file for the guitars, one for the bass, one for the drums and one for the vocals, all separate and 'clean', and all from the actual recordings used on the album track. Using these files, you will have a significantly easier time transcribing/tabbing music, or indeed just learning how to play certain parts.
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