The thermal expansion coefficient of silicate glasses has been correlated w
ith glass structure. It can be expressed in terms of factors for the struct
ural units. The results show that each structural unit contributes to the e
xpansion coefficient with a specific factor the 'differential expansion' of
unit. The factor for a structural unit remains the same over the compositi
on region where it appears. It depends on the type but not the concentratio
n of modifier ion. The factor for Q(4) in any silicate glass is the same as
that obtained for vitreous SiO2. The factors,for structural units in binar
y silicate glasses are mostly the same in mixed modifier glasses.