Quantitatively, the stress is expressed by the "Cauchy traction vector" "T" defined as the traction force "F" between adjacent parts of the material across an imaginary separating surface "S", divided by the area of "S". In a fluid at rest the force is perpendicular to the surface, and is the familiar pressure. In a solid, or in a flow of viscous liquid, the force "F" may not be perpendicular to "S"; hence the stress across a surface must be regarded a vector quantity, not a scalar. Moreover, the direction and magnitude generally depend on the orientation of "S". Thus the stress state of the material must be described by a tensor, called the (Cauchy) stress tensor; which is a linear function that relates the normal vector "n" of a surface "S" to the stress "T" across "S". With respect to any chosen coordinate system, the Cauchy stress tensor can be represented as a symmetric matrix of 3×3 real numbers. Even within a homogeneous body, the stress tensor may vary from place to place, and may change over time; therefore, the stress within a material is, in general, a time-varying tensor field.