The mellotron is now a classic, vintage instrument that made its mark on legendary tracks like The Beatles' "Strawberry Fields Forever" and David Bowie's "Space Oddity". The instrument had a spacey and haunting vibe that hadn't been heard before in the 1960s. It behaved like a pre-cursor to our modern samplers as well. Sound was created by a section of magnetic tape connected to the keys that would playback when a key or keys were depressed, and playback would then stop when the keys were released. This allowed for a variety of sounds depending what instrument or sound was recorded onto the magnetic tape. The mellotron was designed to replicate the sound of the original instrument, but replaying a tape created minor fluctuations in pitch and amplitude.
So a note sounded slightly different each time it was played. How hard a player hit a key affected the sound as well. Another distinction in the sound of the mellotron was that each "sampled" note was recorded in isolation and so the sound had hints of being natural and unnatural at the same time.