Originally Posted by MrInternet
You're right, I was conflating.
IBU's are a subjective measurement of an objective phenomenon... brought about by alpha acids.
Brought about _partially_ by isomerized alpha acids. Oxidized beta acids play a significant role as well. In theory (it'd be tough to get there in practice) you could have 30 IBUs with no alpha acids at all, which would be subjectively very different from 30 IBUS of high-alpha bittering.
Indeed, lambics intentionally go for beta-heavy bittering (by using traditional hops that have a more balanced alpha-beta ratio than modern high-alpha varieties, and then aging them to increase the proportion of oxidized betas).
On deck: Little Bo Pils, Bretta Off Dead (Brett pale)
Secondary: Oude Bruin, Red Sky at Morning (Sour brown ale)
On tap: Saison Duphunk (sour), Amarillo Slim (IPA), Earl White (ginger/bergamot wit)
Bottled: Number 8 (Belgian Strong Dark Ale), Eternale (Barleywine), Ancho Villa (Ancho/pasilla/chocolate/cinnamon RIS), Oak smoked porter (1/2 maple bourbon oaked, 1/2 apple brandy oaked)