The ættir

The Vadstena bracteate and the one from Mariedamm, both around AD 500, carry the full Elder Futhark with dots dividing it into three groups of eight, though the two were struck from the same die, so they are one witness rather than two. The Grumpan bracteate, a little later and damaged in places, shows the same division independently. That makes the grouping one of the few structural facts about the alphabet we can point to directly rather than infer.

The Younger Futhark keeps three groups, but with six, five and five runes rather than eight each. That matters practically: cryptic runes encode a character as a pair of numbers (which group, which position within it) drawn as branching twigs. Get the group sizes wrong and the cipher does not decode.