The problem of those discernible features of the intermediate range order (IRO) which can be attributed to the first sharp diffraction peak (FSDP) observed in the structure factor of many liquid and glassy materials is approached by treating this peak as a distinct feature. It is found, by considering the measured partial structure factors,
S
αβ
(
k
), for molten ZnCl
2
, GeSe
2
, MgCl
2
, NiBr
2
and Nil
2
and the measured total structure factors,
F
(
k
), for glassy SiO
2
, PS
4
and liquid CCl
4
, that the propensity of the FSDP to have a prominent effect on the underlying features of the IRO depends noticeably on the system type. Specifically, the FSDP confers a marked oscillatory character of periodicity 2π/
k
1
(where
k
1
is the FSDP position) on the IRO when the local structural units, which give rise to the density fluctuations on the IRO scale, exist as stable entities for a timescale
τ
≫ 5 × 10
-12
s. The FSDP therefore accounts for the discernible features of the underlying IRO for the viscous glass forming liquids ZnCl
2
and GeSe
2
, for the glasses SiO
2
and PS
4
, and for the molecular liquid CCl
4
. The influence of the FSDP on the IRO is less pronounced for molten MgCl
2
and is negligible for molten NiBr
2
and Nil
2
, both of which have a high cation mobility which leads to a relative instability of the Ni
2+
centred structural units. The effect on the FSDP of temperature and pressure are briefly considered as are the development of the FSDP in molten ZnX
2
(when X is changed from Cl to I to Br) and the minimum size of
r
-space model which is required if the FSDP is to be accurately predicted.
Calculation of the total structure factor and of the layers with their three order parameters for a ternary alloy with spherical symmetry from its invariance under the symmetry group S of exchanges of its components
