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All answers having "Viewpoint" as clue:

4 Character Answers:

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EDGE

SIDE



5 Character Answers:

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ANGLE

SLANT



6 Character Answers:

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ASPECT

STANCE



7 Character Answers:

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OPINION

STERNUM

VANTAGE



11 Character Answers:

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PERSPECTIVE



14 Character Answers:

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REFERENCEFRAME



15 Character Answers:

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SCHOOLOFTHOUGHT