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Hi just brought my first sampler and was wondering what the instructions mean when they say "once you have completed cross stitches add backstitch". Is it to do with finishing off at the back when you have completed a section of cross stitches?

I have just brought my first sampler I have 12 months to complete it. Its for my niece's wedding in July. Just wondered what the term "once you have completed all the cross stitches add the backstitch". I know what cross stitch is obviously but not sure what backstitch means. Any help for a newcomer ...