A model fit for isogeometric analysis is different from a CAD model. The isogeometric model is block structured and each block is, depending on the dimension, a bivariate or trivariate non-trimmed NURBS patch. A CAD model is a set of elementary surfaces such as planes, cylinders, cones and torii, and NURBS surfaces. Many of the surfaces will be trimmed causing gaps and overlaps between the surfaces. A CAD solid is represented by its outer boundary.

The patch structure of a CAD model is often defined from the design history rather than the actual shape of the model. This may result in a model with very many surfaces and where symmetries in the model are not reflected in the patch structure. Moreover, even surfaces that are easily represented as non-trimmed NURBS get a trimmed representation.

Thin plate structures are often planar, but may have a complex boundary and holes. The picture above shows a simple thin plate model. The configuration of the plate is initially defined as one trimmed plane. This surface is a simple CAD model. To remove the trimming, this surface is divided into 4 4-sided surfaces by splitting the trimmed surface from each corner of the outer boundary to the hole. The resulting surfaces can be represented by non-trimmed NURBS surfaces. In the general case, an approximation must be anticipated, but due to the planarity of this model this transformation may be preformed exactly. Finally, the thin plate is constructed by sweeping the surfaces linearily to create 4 NURBS volumes.