Blackboards were their laptopsA little over 3000 BC, Sumerians and Akkadians began scribbling on clay tablets. Writing since antiquity was a form of immortality. Kings and emperors would build enormous steeles/stone obelisks and etch them with their victories. A little later came Wax tablets. Scribes would use stylus to make indentations on wax to write. But these were not particularly effective methods of scribing.
Since 11th century we get mentions of blackboard. Limestone and chalk were readily available all over the world. Black stones, the slate rock, dark wood etc, would be used as canvases to write and draw. Imagine how big a change blackboard could have brought. One could not have drawn per one's whims on tablets of clay and wax as it required immense training and concentration. And the changes made were permanent and could not be restored. Blackboards changed everything. Blackboard paired with chalk pieces for writing, allows the teacher to make as many illustrations as possible. One can wipe the board clean with a cloth and continue. This technology would have been nothing less to a laptop of modern day. One can carry a board and a chalk with him everywhere. You don't waste material like you do in case of paper, papyrus, wax tablet or stone tablet. Earliest of tablets were all from middle east, where a pictorial and hieroglyphic writing form reigned. Not all scripts could have been written easily on them. But chalk and blackboard suddenly made it easy for everyone to write. The materials required for writing were cheap or free. One needed no training at all to scribble on a blackboard. Such it is one of the best inventions or discovery we have ever made. And it has persisted since ages to illustrate students.
With an exception of not being able to preserve the writings, blackboards fulfill all the criteria of being an ideal method of teaching and writing.
(This is my entry for TOW contest).