Printing and paper

Copying becomes cheap.

Movable type

Developed first in China and Korea, and combined in Europe by Gutenberg with metal casting and a screw press around 1450. Book prices fell dramatically within a generation.

Literacy, science and religious upheaval all accelerated afterwards.
Cheap copying made ideas hard for authorities to control.

Later steps

Steam presses, wood pulp paper and photographic reproduction each cut costs again, culminating in digital text with effectively zero copying cost.