To understand sigils, you need to first understand runes. The first runic alphabet was the Elder Futhark runeset created in Germanic/Norse prehistory. The letters were so influential that they still form the foundation of most written word and symbology in the world, definitely the western world. Our letters I, X, and much more never changed from their runic origins.
Runes transformed the world. They gave us codified language and literally provided us with written history. There are other symbolic languages out there, but runes exploded into common usage as a way to get complex ideas across. People wrote out long strings of them into words and sentences, which is what we’re all doing now on reddit. 🙂
The symbols were so transformative that people considered them divine. The first rune, Ansuz (an upward facing triangle with a smaller line in it, which became the modern letter A, the beginning of the alphabet) meant “breath of Odin” who was the head of the pantheon of gods. Runes took on academic, religious, and spiritual meaning.
That brings us to sigils. The early rune users, most notably Vikings, created more powerful symbols called sigils by combining the divine essence of two or more runes into more complex symbols. For example, they’d combine runes of strength and health to make protective wards for shields. They’d combine strength with fire to make a rune for success in battle, which would be inscibed in warships or weapons. They would make symbols to ward from “the evil eye” or to curse enemies.
Two of the most famous sigils are vegvisir and aegishjalmur. The first was meant to protect and guide travelers through bad weather and prevent getting lost. The second was to instill “the fear of god” into rulers so they would not abuse their power. Sigils were far more complex and powerful than mere runes.
The most popular sigil in modern times is in your phone settings right now. The Bluetooth logo is a combination of the younger futhark runes for H and B. You can read more about it here:
https://www.creativebloq.com/news/bluetooth-logo-secret
By the way, my upcoming fantasy series deals heavily with runes and sigils. 🙂