THE RECORD
What changed
Song texts record directional magnetic needles and Bi Sheng's ceramic type; later forms adapted to materials and scripts.
EVIDENCE
What remains
Shen Kuo's Dream Pool Essays, navigation texts, surviving type and books preserve early use.
WHY IT MATTERS
Why it matters
Direction and letters could be detached from fixed routes and blocks, then recombined.
OPEN QUESTION
Where interpretation begins
Invention, navigation and transmission dates differ; movable type did not immediately replace woodblocks in East Asia.