Does Gimli have a single-sided or a double-sided axe? (Please answer only with quotes from the Lord of the Rings or other Tolkien books, I've done enough speculation myself.)
In The Two Towers, book 3, chapter 6, The King of the Golden Hall, we find this passage:
'Not alone!' said Gimli, fingering the blade of his axe.
It doesn't conclusively state, but certainly implies, it's a one-bladed axe. Otherwise it wouldn't be the blade.
Another possible reference would be the works of Alan Lee, John Howe, Ted Nasmith and other illustrators whose works are widely seen as definitive, but I don't think Tolkien himself lived to see most of them, so they can't really be seen as proof. This is an interesting page about various Tolkien illustrators, some loved by the man himself, but I couldn't find any of Gimli or his axe there.
Book 2, Chap 3: "The Ring Goes South"
Gimli the dwarf alone wore openly a short shirt of steel-rings, for dwarves make light of burdens; and in his belt was a broad-bladed axe
And see this Wikipedia entry.
I don't believe Tolkien ever clearly said if Gimli's axe was single-edged or double-edged. I found two quotes in addition to those provided in the earlier answers that may suggest Tolkien had a single-edged axe in mind.
I am not saying that these constitute proof, just a small hint.
When the Fellowship reaches the entrance to Moria and is looking for the door, Gimli taps the stone with his axe. He would probably not tap the stone with the sharpened edge of his axe, so this action would be most natural if done with the unsharpened end of a single-edged axe.
Gimli was wandering about, tapping the stone here and there with his axe.
The Lord of the Rings Book 2, Chapter 4: A Journey in the Dark
When Gimli and Legolas meet again after the Battle of Helm's Deep, Gimli says
‘Forty-two, Master Legolas!’ he cried. ‘Alas! My axe is notched: the forty-second had an iron collar on his neck. How is it with you?’
The Lord of the Rings Book 3, Chapter 8: The Road to Isengard
If the iron collar had notched only one of the two edges of his axe, Gimli might have said that, rather than that his axe was notched.
I found this quote:
They turned and ran. At that moment some dozen Orcs that had lain motionless among the slain leaped to their feet, and came silently and swiftly behind.
Two flung themselves to the ground at Éomer's heels, tripped him, and in a moment they were on top of him. But a small dark figure that none had observed sprang out of the shadows and gave a hoarse shout: Baruk Khazâd! Khazâd ai-mênu! An axe swung and swept back.
Two Orcs fell headless. The rest fled.
The Two Towers - Helm's Deep
Which might suggest that Gimli had a double bladed axe. With one forward swing and a back swing he was able to decapitate two orcs. Gimli either:
- Took off both heads on the initial swing, and the sweep back is superfluous.
- Took off one head on the initial swing and on the sweep back took off the second head with the axe's second blade.
- Took off one head on the initial swing and on the sweep back turned the aze and took off the second head with the axe's single blade.