  Relativistic iron K$\alpha$ spectral emission line from the inner disc of a
neutron star low-mass X-ray binary (LMXB) was first detected in 2007. This
discovery opened up new ways to probe strong gravity and dense matter. The past
decade has seen detections of such a line from many neutron star LMXBs, and
confirmation of this line from the same source with several X-ray satellites.
These have firmly established the new field of relativistic disc line from
neutron star systems in only ten years. Fitting the shape of such a line with
an appropriate general relativistic model provides the accretion disc inner
edge radius to the stellar mass ratio. In this review, we briefly discuss how
an accurate measurement of this ratio with a future larger area X-ray
instrument can be used to constrain neutron star equation of state models.
