https://en.wikipedia.org/wiki/IBM_PC_Convertible has all the details on that machine you could want.

Incredibly forward-thinking modular architecture. Keyboard, memory, drives, serial port, parallel port, even the screen could be replaced just by the turn of a lever or a push of a button.

Fantastic keyboard, even by today's laptops standards.

Ate batteries like M&Ms. I almost always kept it plugged in.

At the time, running it off the pair of 720k floppies was fine. I believe there was a hard drive option, but I never saw it.

Its biggest weakness was the screen. There were backlit and CRT options, which were better and you could just pop off and in.

The screen was grayscale CGA, but there was a TSR called SimCGA which would translate, so you could run EGA programs.

Back in the early 80’s, Radio Shack made the TRS-80 Model 100 laptop. It ran for 20+ hours on 4 AA batteries.

A few years later, Psion came out with a series of small devices that ran on 2 AA batteries and got 30+ hours of runtime.

With modern electronics and displays, could something like a model 100 be made that could run for hundreds or even thousands of hours on 4 AA batteries?

One interesting solution to this space was GRiD making their battery and power supply the same size and form-factor --- when one was working at a desk, to save space the battery could be removed and placed in a separate charger, while the cord from the power supply to the computer was removed, and it was then placed in the battery compartment and connected to the wall, powering the device.

The transformer was outboard on this machine, like a modern computer. It had its own Velcro compartment in the carrying bag.

Originally, there were portables (sometimes referred to as "luggable"), like the Osborne or Compaq Portable series. The early models were the size of a small suitcase or large briefcase and contained a CRT screen, usually with a detachable full-size keyboard.

Later, portables ditched the CRT in favour of (very readable) gas plasma displays, allowing for greatly reduced depth. The final models were roughly the size of two shoeboxes stacked on top of each other, and were sometimes referred to as a "lunchbox".

Laptops took the opposite approach, reducing height rather than depth. This IBM 5140 was a good early example, but I think the first might actually have been the Data General DG-1 in 1984.

They had a flat screen (usually passive matrix) with a hinge directly behind the keyboard. About a third of the case stuck out behind the hinge, and typically housed the battery (usually lead acid), floppy disk, and HDD.

Unlike the previous luggables, they could just about be used on a lap for short periods. They often weighed around 5-6kg, though, so most of them will have been used on a desk or table almost all of the time.

Notebooks came a few years later (1989-ish), with the NEC Ultralite, Toshiba Dynabook, and Compaq LTE leading the way, and were distinguished by being smaller still - the size of a ream of A4 paper - and having the hinge right at the back of the machine.

They tended to be lower-powered (8086 CPUs rather than 286 or 386), and initially only had a FDD as they were too small to fit a full-height 3.5" hard drive. They weighed around 2-3kg, so actually could be used on a lap.

The limitations of the smaller models evaporated quickly, and notebooks had almost completely taken over by the mid 90s. The last lunchbox portable was probably the Compaq 486 in 1992. There were still a few rugged or workstation laptops being produced right up to the end of the decade, but they were pretty rare by that point.