One of the carved Jelling stones erected by Harald Bluetooth in memory of his parents.
You've heard of bluetooth, haven't you? You know, the way that electronic devices talk to each other wirelessly when they're near each other. I bet you didn't know that it was named after a Viking! If you look again at the symbol, you'll see it's a combination of the runes for H and B, the initials of a Viking called Harald Bluetooth.
The exact story behind this has become a bit of a myth in itself. Back in the mists of the 1990s, when bluetooth technology was invented, the story goes that the developer was reading a book called "The Long Ships", which features Harald Bluetooth. So he nicknamed the new technology 'bluetooth', just until someone came up with something better. But the nickname stuck!
Who was Harald Bluetooth anyway, and did he actually have blue teeth? Well, he was King of Denmark starting about twenty or thirty years after Saving the Unicorn's Horn takes place, until 986, and King of Norway too at some point. Whether or not he had blue teeth has also become a bit of a myth, because of the various possible translations of the nickname from the original 'blátǫnn' in Old Norse.
If he really did have a blue tooth or teeth, it may have been because:
one of his teeth was dead and therefore bluish or black in colour ("blue" meant 'dark' in Old Norse);
or perhaps he used a technique that other warriors used, in which they filed horizontal stripes across their front teeth and coloured them with dyes to make themselves look more frightening in battle. There's a plant called woad which was used to make blue dye (remember Braveheart?), so perhaps Harald Bluetooth striped his teeth with woad to terrify his enemies.
On the other hand, it may well be that the nickname 'blátǫnn' had nothing at all to do with teeth - it translates as 'dark chieftain' or something similar (in which case, where did the 'tooth' bit come from?!) So perhaps Harald just had dark skin or hair, and a gleaming white smile!
Ericsson's cheeky cartoon based on the same stone.
In any case, it's rather funny to think that a symbol that many of us see everyday is related to a long-dead Viking, with or without blue teeth!