The behaviour of all reconstituted and natural clays is determined by their fabric and bonding. A new classification of clays is proposed in this paper on the basis of standard penetration test (SPT), the geological history and the one- dimensional compression of the clay in the I_V - σⁱ_v plane. Although the eight clay types defined in the classification have different origins, fabric and bonding, they all have either a syn-sedimentation or a post- sedimentation structure. The definition of these clay types is taken as a starting point for the construction of a general framework of behaviour of clays. In this research results of laboratory and field investigations of a very stiff Tehran silty clay in the natural and reconstituted states including SPT, Oedometer, Scanning Electron Microscopy and polarizing microscopy are presented. The structure of the Tehran silty clay is strongly influenced by bonding, calcium carbonate content and weathering intensity. This soil is a very stiff to hard clay which geologically is over-consolidated. Consolidation curve of soils lies close to the ICL line. Hence the Tehran silty clay is a type 4 clay but, at certain depths where the yield stress ratio is greater than 1, the Tehran silty clay becomes a type 7 clay.