A patsy is an element used as a placeholder or temporary substitute to allow building to continue until the correct element can be found. Most often a patsy is the right element in the wrong colour. Using a highly contrasting colour reminds the builder to change the piece out and makes it easier to spot when the correct element is obtained.
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The definition of patsy 1 is a dupe or scapegoat; someone easily framed for a mistake or transgression. A notable modern use of the term was the character Patsy the servant, so named, of King Arthur in the film Monty Python and the Holy Grail. The character's principle purpose was to take abuse and bear the burdens of the king.