Before it
Chain of thought needed hand-written worked examples in the prompt.
The problem
Writing exemplars for every task is laborious and task-specific.
What it introduced
A single phrase asking the model to think step by step, with no examples, unlocks most of the same gain.
Why it matters
It showed the reasoning behaviour was already in the model, waiting for a cue. Cheap, universal, and much copied.
What modern models inherit
The instruction to reason before answering.
What came next
Absorbed into system prompts everywhere, then trained in directly by reasoning models.