Have a client with a house built in the '20s, originally knob and tube
wired but (partly) converted to more modren wiring methods. Basement
ceiling is exposed and where the majority of the house wiring is. Lots
of strange, non-standard and some clearly improper wiring exists the
"flying" splices between K&T wiring and NM or armored cable (no junction
box), and one place where someone just ran a single insulated wire (THHN
or whatever) as a neutral from one place to another. Plus exposed K&T
runs going everywhere, one right next to the hot-water shutoff valve.

So I've figured out pretty much what goes where and how to replace it
(mostly with runs of Romex, since it's not an occupied space).

But there's one thing about the old wiring that's puzzling to me.
Usually K&T runs are done with pairs of wires (Hot & neutral) running
alongside each other, and in most cases that's followed he a run up
into a wall will have a pair of wires going up into the subfloor.

But there are a couple of places where a *single wire* is routed up out
of the basement into the house above. In these cases, the wires are all
hots. No corresponding single neutral wire anywhere nearby. Presumably
the neutral side of the circuit is tied to the neutral of another pair
of wires.

This, of course, makes it difficult to map these circuits, since I can
only assume that the neutral connection is made to the neutral wire
corresponding to the hot wire. That's my operating assumption, anyhow:
my plan is to simply replace these single-wire runs with Romex, using
only the black wire. (Hmm, wonder if they make 12-1 Romex w/o ground?
Probably not.)

Anyone ever seen this situation in an older house?

--
Found--the gene that causes belief in genetic determinism