Golden Rule 8 : Ensure Tangency across Centre-lines
If a design has some symmetry, we need to pay particular attention to those surfaces which touch or cross the centre-line, to make sure that the result is smooth.
For example, if we build just one half of a design, and then mirror it to complete the model, we need to achieve tangency and avoid unwanted 'peaks' or 'troughs' between the two mirrored halves.
