The main motif of the quilt should be echoed or repeated in the border. Strong lines should be repeated and lighter designs are there in overtones. Example - a ribbon and bow border would look good on a dresdan plate but ridiculous on a log cabin.
A well-proportioned border is when it is a little wider than each of the center blocks. A border that is too narrow makes the central part of the quilt overly conspicious, but a frame that is too wide will dwarf the significance of the central pattern.
There are lots of web sites that tell how to apply a border, make good corners or fancy edges. Here are a couple pictures of border ideas. I like to piece borders.
adapted from: The Standard Book of Quilt Making and Collecting by Marguerite Ickis pp 155-165
each row is a different border pattern
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