Spelling of btree throughout the regular doc and arch guide alike is inconsistent. It seems like a strong majority (70%) of uses in the regular reference doc are "btree". Approx 20% are "Btree", the rest either b-tree/b+tree/B-tree/B+tree. It's a closer race in the arch guide, but "btree" (40%) is still winning against "Btree" (25%) and "B-tree" is (20%).

I think we should standardize on one, the obvious choice is "btree", except on perhaps a few occasion when we talk about the actual implementation flavor being a "B+tree".