  {The \ion{Na}{x} X-ray lines between 10.9 and 11.2~\AA\ have attracted little
attention but are of interest since they enable an estimate of the coronal
abundance of Na to be made. This is of great interest in the continuing debate
on the nature of the FIP (first ionization potential) effect. } {Observations
of the \ion{Na}{x} lines with the Solar Maximum Mission Flat Crystal
Spectrometer and a rocket-borne X-ray spectrometer are used to measure the
Na/Ne abundance ratio, i.e. the ratio of an element with very low FIP to one
with high FIP.} {New atomic data are used to generate synthetic spectra which
are compared with the observations, with temperature and the Na/Ne abundance
ratio as free parameters.} {Temperature estimates from the observations
indicate that the line emission is principally from non-flaring active regions,
and that the Na/Ne abundance ratio is $0.07 \pm 50$\%.} {The Na/Ne abundance
ratio is close to a coronal value for which the abundances of low-FIP elements
(FIP $< 10$~eV) are enhanced by a factor of 3 to 4 over those found in the
photosphere. For low-temperature ($T_e \leqslant 1.5$~MK) spectra, the presence
of \ion{Fe}{xvii} lines requires that either a higher-temperature component is
present or a revision of ionization or recombination rates is needed. }
