  A family of mixed finite elements is proposed for solving the first order
system of linear elasticity equations in any space dimension, where the stress
field is approximated by symmetric finite element tensors.
  This family of elements has a perfect matching between the stress components
and the displacement.
  The discrete spaces for the normal stress $\sigma_{ii}$, the shear stress
$\sigma_{ij}$ and the displacement $u_i$ are
  $\operatorname{span}\{1,x_i\}$, $\operatorname{span}\{1,x_i,x_j\}$ and
$\operatorname{span}\{1\}$, respectively, on rectangular grids.
  In particular, the definition remains the same for all space dimensions. As a
result of these choices, the theoretical analysis is independent of the spatial
dimension as well. In 1D, this element is nothing else but the 1D
Raviart-Thomas element, which is the only conforming element in this family. In
2D and higher dimensions, they are new elements but of the minimal degrees of
freedom. The total degrees of freedom per element is 2 plus 1 in 1D, 7 plus 2
in 2D, and 15 plus 3 in 3D.
  The previous record of the least degrees of freedom is, 13 plus 4 in 2D, and
54 plus 12 in 3D, on the rectangular grid.
  These elements are the simplest element for any space dimension.
  The well-posedness condition and the optimal a priori error estimate of the
family of finite elements are proved for both pure displacement and traction
problems. Numerical tests in 2D and 3D are presented to show a superiority of
the new element over others, as a superconvergence is surprisingly exhibited.
