Let be a partially ordered set, and let . If , then is said to be between and . If is between and and , then is strictly between and .
This entry contributed by Matt Insall (author's link)
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Insall, Matt. "Strictly Between." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/StrictlyBetween.html