SL block is an octocube that may interlock with other SL blocks to form infinite variations of stable structures. The property of self-interlocking makes SL block expressive to explore the beauty of symmetry, which has been regarded as an essence of art and mathematics by many. This paper describes a mathematical representation that maps polynomial expressions to compositions of SL blocks. The use of polynomials, functions and hierarchical definitions simplifies the creation, communication and manipulation of complex structures by making abstractions over symmetrical parts and relationships. The discovery of SL block and its mathematical representation lead the way towards the development of an expressive language of forms and structures which is at the same time, rich and compact, free and disciplined.