This chapter sets out to introduce one of the most popular two-dimensional continuum elements used by finite element practitioners. It will deal exactly with second order displacement fields and so first order or linearly varying stress fields. If, as is mostly the case, the displacement and stress fields are of higher order, then the element gives a numerical approximation, error being minimized if its size is progressively decreased and its shape maintained as near square as possible. However the complex problem of modelling is beyond the scope of this text.