@dave r said:
The trick is to join both pieces of molding so either the high points or the low points on the edges meet.
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Then miter them.
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Flip one of the lengths of molding end for end and adjust the position so it looks more like a continuous run of rope. around the corner.
The "real" trick is to do it in situ. When a number of inside and outside corners have to match.
Get the Douzukinoko saw out! For numerous 1/32 inch cuts to adjust the length.