I would only add, having been the partner of someone with bbd for many years, that, while the episodes of extreme emotion dysregulation in bpd may be more sort of sudden, I guess, and brief, some folks with bp 1 have far more mood instability/lability at times during their/our illnesses than others. For example, I recently had a period of maybe 36ish hours in which I was extremely hypo, out of nowhere. Then, it vanished.

So, in practice, it can, in fact, be quite difficult to sort out the purely mood-related issues in these two illnesses. But as WC wisely states, there are a whole universe of other things that distinguish the two.