  For a singular variety X, an essential step to determine its smoothability
and study its deformations is the understanding of the tangent sheaf and of the
sheaf T^1_X:=ext^1(Omega_X,O_X).
  A variety is semi-smooth if its singularities are \'etale locally the product
of a double crossing point (uv=0) or a pinch point (u^2-v^2w=0) with affine
space; equivalently, if it can be obtained by gluing a smooth variety along a
smooth divisor via an involution with smooth quotient.
  Our main result is the explicit computation of the tangent sheaf and the
sheaf T^1_X for a semi-smooth variety X in terms of the gluing data.
