By definition, all commercial records pressed before the introduction of stereo LP records in 1957 were monaural (mono), which means they were only able to output one signal channel to speakers or headphones. The driving force behind these two-channel “stereophonic” LPs—a concept that had been tinkered with since the 1930s—was the idea that recorded music sounds better when each ear is presented with a different element of the overall sound. In the early ’60s, only adults with deep pockets could afford a state-of-the-art “hi-fi” stereo system with two speakers. The biggest consumers of pop music, kids and teenagers, made do with cheap mono record players with only one speaker. Stereo records also cost a dollar more than mono discs. For these reasons, artists that appealed to adults, like Frank Sinatra, were more likely to put out stereo recordings than pop stars. That all began to change, though, in 1966, when inexpensive stylus cartridges allowed stereo records to be played on mono turntables. Most of the popular rock bands of the time, like the Beatles and the Rolling Stones, put very little energy into their stereo recordings, focusing all their energy on performing, mixing, and mastering their mono tracks. Stereo mixes were made almost as an afterthought. Take 1967’s “Sgt. Pepper’s Lonely Hearts Club Band,” which is now considered one of the greatest stereo experiences on vinyl. At the time, the Beatles and their world-renowned producer George Martin lavished great time and attention on the mono recording. The stereo mix was left to a secondary producer named Geoff Emerick, who tossed off the task in three hours. Later, George Harrison would insist, “You haven’t heard ‘Sgt. Pepper’ if you haven’t heard it in mono.” Similarly, Motown Records chief Berry Gordy, Jr. put his top engineers on the mono recordings, while trainees were left to work on the stereo mixes. Phil Spector, widely considered one of the greatest pop producers of all time, mixed his best work in mono. While consumers eventually started to accept and even enjoy the special effects afforded by stereo recordings, like echoes, extra guitar solos, and speaker-to-speaker panning, many audiophiles today insist that original mono recordings sound far better than their stereo counterparts. For mono mixes, the musicians usually played together in the same room, vibing off of one another. When they recorded in stereo, though, they wore headphones to make sure they got their individual tracks right. Additionally, many mono LPs were produced from entirely different recordings than the stereo releases, so the differences between the mono and stereo versions of some songs, and even some albums, can be quite noticeable. As stereo records became more common, labels with lots of mono recordings on their hands went back to the two-channel master tapes laid down in the studio and re-released the same albums in stereo. Other labels that were keen to jump on the stereo bandwagon, but who lacked two-channel masters, simply remixed their mono recordings by separating the vocal and instrumental tracks and placing them in separate channels, often with a split-second delay. This “electronically reprocessed stereo,” or “fake stereo,” was sometime used on stereo reissues of old recordings, to horrible effect. Today, the most valuable mono LPs are those produced in the late ’60s when mono was being phased out (1968 in the U.S., 1969 in the U.K., 1970 in South America). Collectors are particularly interested in “Sgt. Peppers,” the Beatles’ “The Beatles” (a.k.a. “The White Album”), the Stone’s “Their Satanic Majesties Request,” Jefferson Airplanes’ “After Bathing at Baxter’s,” The Monkees’ “The Birds, the Bees, and the Monkees,” and the Who’s “The Who Sell Out.” In the 1990s and 2000s, as mono nostalgia surged, many of these mixes were released on CD, often sold along with the stereo album in a two-disc package. These include the Crazy World of Arthur Brown’s 1968 self-titled debut, the Who’s “A Quick One: Happy Jack,” Jefferson Airplane’s “Surrealistic Pillow,” 1967’s “The Velvet Underground and Nico,” The Pretty Thing’s “SF Sorrow,” and many early Bob Dylan albums. Even early psychedelic records by Pink Floyd, a late '60s band known for exploring and exploiting the sonic capabilities of stereophonic sound, have been reissued in the mono format, including “The Piper at the Gates of Dawn” and “A Saucerful of Secrets.” Music lovers who listen to mono and stereo LPs side-by-side will note the subtle—and sometimes obvious—discrepancies between recordings. Instruments in the background might emerge, while others might fade out entirely. Voices not heard on stereo might appear on mono versions, while completely different instrumental solos, intros, or outros are sometimes used. Mono recordings can be slower and more sparse, faster and more aggressive, or sonically more dense than stereo recordings of the same songs. Some of the many mono LPs that are noticeably different from the more-common stereo pressings include Jeff Beck’s “Truth,” Cream’s “Wheels of Fire,” Jimi Hendrix Experience’s “Axis: Bold as Love,” Love’s self-titled album, and the Velvet Underground’s “White Light/White Heat,” which features a secret track on the stereo pressing: If you turn off the stereo channel while listening to “The Gift,” the vocal channel drops out, revealing a mono instrumental titled “Booker T.” Another record of particular interest to collectors is Van Morrison’s “Blowin’ Your Mind,” released in mono by Bang Records in 1968. The famous line "making love in the green grass," has been edited out, replaced with a repeat of "laughin' and a-runnin', hey hey.” The missing line appears on subsequent stereo pressings, but using a different mix of the song. The mono LP also has a mislabeled catalog number, “BLB 218,” when it should be “BLP 218.” A mono LP's worth is usually determined by how many were produced. For example, even though the Rolling Stones have insisted that mono is the only way to listen their early records, the stereo LP of their 1965 album “Out of Our Heads” is rarer and more valuable than the mono pressing. Conversely, the Beatles’ 1968 album “Yellow Submarine” is more sought-after in mono. Condition is a factor, too—as most mono LPs belonged to kids, the ones that survived the ’60s tend to be in dreadful condition. Interestingly, stereo 45s weren’t common until around 1969. Teenagers were the main customers of pop singles, and most of them only had access to older turntables or portable players like the Dansette that couldn’t be fitted with new stereo stylus cartridges. By 1970, mono LPs were a thing of the past, revived only on very rare occasions by groups such as Dr. Feelgood, who, in 1975, released a mono version of “Down at the Jetty” to capture that vintage single-channel sound.