Now lets apply some of the key concepts of earlier posts. In practical terms, if the imperfect timber we are using has two relatively square surfaces it is easier to imagine that the perfect timber inside begins on these two surfaces(these are the reference faces) and the remaining perfect faces lie within the remaining imperfect timber. If we align the reference faces of the timber with the reference planes of the building then the important dimensions and functions of the building/structure will be "fixed".
Once the reference planes are identified it should be fairly simple to identify them on the timber right? You can just pick any face to be the top reference face right? Well No. Picking the correct surface will make laying out joinery simpler, will make the frame stronger, and ensure that the final frame dimensions match the drawing. The essential concept here is to properly match the timber’s natural characteristics (both strength and aesthetic traits) to the requirements previously identified. Timbers, all wood really, have natural strength characteristics that are derived from their growing conditions and how the timber was sawn out of the log.