Where only one of both bounding surfaces are curved.
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Cross-section of a trough, a wheelbarrow, some rooves.
Bedding you can flip inside out for different colors and designs.
You haven't told us how long the trough is, or how deep the water is in it. If it's full to the rim ... 2-ft deep ... then there's 44.88 gallons for every foot of the trough's length.
The trough is the bottom and the crest is the top. The whole vertical length of the crest, trough, and crest are called a wavelength. See related link for a diagram.
The zero point for longitude runs trough (is in) Greenwich, England.