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Left-sorted Bongard Problems have the keyword "both" on the OEBP.
The archetypal example is "rhombuses vs. rectangles".
Notice "rhombuses vs. rectangles" could alternatively be interpreted as "not rectangles vs. not rhombuses"; by this less natural interpretation, a square would fit on neither side (keyword neither) rather than both.
In fact, for any Bongard Problem solution "A vs. B", there are three alternative solution descriptions: "A vs. not A", "not B vs. B", and "not B vs. not A". These are not necessarily just different wordings of the same answer. For example, "rhombuses vs. not rhombuses" and "not rectangles vs. rectangles" differ on where they would sort a square. (This discrepancy between "A vs. not A" and "B vs. not B" occurs whenever "A vs. B" does not sort all relevant cases. See the keyword allsorted.)
"Is a rhombus" and "is a rectangle" are what are on the OEBP called "narrow" patterns, while "is not a rectangle" and "is not a rhombus" are not. (See keywords left-narrow and right-narrow for more explanation.) | 
