While he was a professor at Oxford, he became really good mates with the author CS Lewis.
You may recognise his name - he wrote the Narnia novels.
Tolkien showed him and some other friends his rough drafts of what was to become to Lord of the Rings trilogy.
They helped him by telling him what they thought of the complicated work, which took 17 years to complete!
The group of friends became known as the Inklings.