Know your wedges from your side fills
The 1930s were a great time for popular music. Amplification was invented.
Now, the singer could be boosted up in level and louder instruments used in the band, and more of them.
All through the 1930s and 1940s, and a large part of the 1950s, this continued - the singer was amplified and the rest of the instruments were loud enough. (The acoustic guitar, being a rather quiet instrument compared to trumpets and saxophones, was often amplified too, but only enough to make it audible).
Then came rock 'n' roll.
Suddenly, amplified instruments were all the rage. So a typical rock 'n' roll band would have amplification for the singer - a vocal PA - and individual amplifiers for the guitars. The drums remained acoustic.
Already there was a problem. Although the guitar players and drummer could hear themselves perfectly clearly, the singer could not hear himself or herself very well. The vocal PA loudspeakers have to fade forwards and not a lot of sound comes out of the back. It was common to position and angle the PA cabinets so that at least some sound would leak back to the singer.
Obviously the answer is to provide speakers on stage that are there solely for the benefit of the singer - monitor loudspeakers. Originally this would be any cabinet that was to hand. Eventually it developed into the wedge-shaped monitor that is familiar today.
At this point, loud music became fashionable. Extremely loud, like we currently prefer music to be.
Now the drummer would be amplified too. First by a single mic, then by the one-mic-per-drum plus overheads approach that is still current.
Now everyone needed a wedge monitor.
But many singers didn't like to be rooted to the one position where they could hear their wedge monitor. The answer was to place additional cabinets at the sides of the stage so the entire stage could be filled with sound. Naturally, these loudspeakers are known as side fills.
The problem is that now there are so many monitor loudspeakers on stage, the sound can get very confused. It takes a skilful monitor engineer to control all of this and keep the band happy.
In fact, you could say that the solution to the problem has caused just as many problems in itself. So now we see a move away from too many monitors and towards in-ear monitoring.
Without doubt though, further progress in monitor loudspeakers would be very desirable. Perhaps one day wedges and side fills will be history.