Until the early 1980s most monitors were terminals. They were boxy video display terminals (VDTs) combined with an attached keyboard. A terminal could be configured to work with just about any computer on the market. (Not that there was a wide selection of personal computers for you to choose from.)

Terminals were attached to computers by a serial interface. In those days, the VDT was commonly referred to as a CRT (Cathode Ray Tube).

Before DOS, the dominant operating system (OS) for 8 bit computers was CP/M (Control Program for Microprocessors). Early CP/M machines were originally designed to use separate memory-mapped video display devices and discrete keyboards that plugged into the machines not unlike video display cards used later. The most well known was the VDM-1. Terminal manufacturers recognized this lost market and began to market mainframe and mini-style terminals to the CP/M community. The sales pitch of just like a real (mainframe at the time) computer paid off. CP/M computers soon used terminals almost exclusively.

Apple II computers and the early game machines (such as those made by Atari, Coleco or Nintendo) hooked to a monitor not a terminal. (The Apple II was built with a keyboard as part of the system. All that was missing was a monitor once the Apple II was plugged in).

These monitors unlike terminals looked like television sets without the tuner. In some cases they actually were television sets. (Many early computers such as the Commodores Vic 20, 64 and 128, could be used with any television set with a special RF adapter that hooked to the antenna of the TV).

Then IBM came out with PC-DOS computers, which were dubbed three-piece computers. One explanation according to a prominent used car dealer Moonie Bronstein was that many of the early marketers / hucksters advising the techies of the early computer era ,had their start in the competitive world of auto sales where such terms as 3 piecers and 4 piecers were popular marketing and sales terms. Other explanations for this marketing term was because the computers included three main components i.e. the monitor, the keyboard and the CPU box.

Ironically, when the IBM PC-DOS computers arrived on the scene with separate monitor and keyboard the monitor connected directly to the computer. Just like the earliest personal computers) through a display device connection. These new monitors used video cards that were either IBM monochrome (MDA), IBM color graphics cards9 CGA), or Hercules (the first third party ad on cards).