Centre bars are smaller than the stretcher bars because they fit inside the rectangle, whereas the stretcher bars are sized based on the outside of the rectangle they produce.

But we have already figured out the sizes that fit and they are labelled as the size needed for the rectangle, rather than the actual length of the bar. So a 50cm centre bar is not actually 50cm long itself, rather it is made to fit inside a 50cm stretcher bar.

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