  It has become commonplace is astronomy to describe the transverse coarse
structure of jets in loosely defined terms such as "sheath" and "spine" based
on discussions of parsec scale properties. But, the applicability, dimension
and prominence of these features on sub-lt-yr scales has previously been
unconstrained by observation. The first direct evidence of jet structure near
the source in M\,87 is extreme limb brightening (a double-rail morphology), 0.3
- 0.6 mas from the source, that is prominent in observations with high
resolution and sensitivity. Intensity cross-cuts of these images provide three
strong, interdependent constraints on the geometry responsible for the
double-rail morphology: the rail to rail separation, the peak to trough
intensity ratio and the rail widths. Analyzing these constraints indicates that
half or more of the jet volume resides in a thick-walled, tubular, mildly
relativistic, protonic jet only $\sim 0.25$ lt-yr (or $\sim 300$ M, where M is
the central black hole mass in geometrized units) from the source. By contrast,
the Event Horizon Telescope Collaboration interprets their observations with
the aid of general relativistic magnetohydrodynamic simulations that produce an
invisible (by construction) jet with a surrounding luminous, thin sheath. Yet,
it is shown that synthetic images of simulated jets are center brightened 0.3 -
0.6 mas from the source. This serious disconnection with observation occurs in
a region previously claimed in the literature to be well represented by the
simulations. The limb brightening analysis motivates a discussion of possible
simulation modifications to improve conformance with observations.
