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When drilling mortises for chair arms, such as those in a ladderback chair, it can be difficult to accurately center your bit.
But wrap a string around the rear post at the mortise location and then around the front post, and finding the center gets a lot easier.
By resting the bit extender on the front post and using the string to visually locate center while drilling, you ensure the mortise will also be angled correctly. –David Douyard
