Most wood boards contain knots. Knots are known as the most weakening factor affecting mechanical properties and the major cause of drying deformation. Some useful mathematical models were developed in this article for later computer simulation on the behaviours of knotty boards in service and in processes. Geometrical features of knot wood including growth angle, position and three dimensional shape were modelled. All sawn patterns and cutting positions were generated. Formulae and modelling steps were introduced and geometrical features of knots were discussed in detail using projective geometry.