If is irreducible, then roots in have at most two different lengths. Here I mean actual geometric lengths, as measured by the inner product, not the “length” of a Weyl group element. Further, any two roots of the same length can be sent to each other by the action of the Weyl group.
Let and be two roots. We just saw that the -orbit of spans , and so not all the can be perpendicular to . From what we discovered about pairs of roots, we know that if , then the possible ratios of squared lengths are limited. Indeed, this ratio must be one of , , , , or .
If there are three distinct root-lengths, let , , and be samples of each length in increasing order. We must then have and , and so , which clearly violates our conditions. Thus there can be at most two root lengths, as asserted. We call those of the smaller length “short roots”, and the others “long roots”. If there is only one length, we call all the roots long, by convention.
Now let and have the same length. By using the Weyl group as above, we may assume that these roots are non-orthogonal. We may also assume that they’re distinct, or else we’re already done! By the same data as before, we conclude that . We can replace one root by its negative, if need be, and assume that . Then we may calculate:
We may note, in passing, that the unique maximal root is long. Indeed, it suffices to show that for all . We may, without loss of generality, assume is in the fundamental doman . Since , we must have for any other . In particular, we have and . Putting these together, we conclude
and so must be a long root.