Jim wrote:
Still thinking about VNE and whether it is usually stated as a TAS
rather than an IAS (one must read the POH to be sure, of course).

I've gotten the notion, probably from comments I've not understood
very well, that the "coffin corner" is the intersection of stall speed
as an IAS indication on the airspeed indicator, and VNE understood as
a TAS and thus occurs at a decreasing IAS with altitude.

I guess the consequence of this notion is that as aircraft altitude
goes up the stall speed TAS goes up to ultimately bump into
the VNE TAS. If VNE is published as an IAS, like stall speed,
then stall speed and VNE would never converge.

VNE is due to two effects, compressibility (TAS affected), and force of
the wind (IAS). Stall speed is an IAS thing.

The corner is when you can go no slower (stall/IAS) and no faster
(compressibility/TAS). Bad place to be.