The "I" in "Atlas I" can cause confusion, as all previous Atlas rockets were designated using letters, ending with the Atlas H, however subsequent rockets were designated using roman numerals, starting with the Atlas II.
The trio of rocket engines used in the first stage of the American Atlas I and Atlas II launch vehicles, arranged in a "row", used parallel staging in a similar way: the outer pair of engines existed as a jettisonable pair which would, after they shut down, drop away with the lowermost outer "skirt" structure of the booster, leaving the central "sustainer" engine to complete the first stage's engine burn towards apogee or orbit.
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