The terms A-side and B-side refer to the two sides of 78, 45, and 33​⁄₃ rpm phonograph records, or cassettes, whether singles, extended plays (EPs), or long-playing (LP) records. The A-side usually featured the recording that the artist, record producer, or the record company intended to receive the initial promotional effort and then receive radio airplay, hopefully, to become a "hit" record. The B-side (or "flip-side") is a secondary recording that has a history of its own: some artists released B-sides that were considered as strong as the A-side and became hits in their own right. Others took the opposite approach: producer Phil Spector was in the habit of filling B-sides with on-the-spot instrumentals that no one would confuse with the A-side. With this practice, Spector was assured that airplay was focused on the side he wanted to be the hit side.