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Euclid’s Elements – Book 1 – Definitions, definition XXII, simply says:
“A quadrilateral figure is one which is bounded by four sides.”
So any four sided shape is a quadrilateral. The shape I use in this piece is a form of a quadrilateral that Euclid describes as a kite. Yes, it’s the shape your thinking of, like the traditional flying kite shape.
In Euclidean geometry, a kite is a quadrilateral whose four sides can be grouped into two pairs of equal-length sides that are adjacent to each other. In contrast, a parallelogram also has two pairs of equal-length sides, but they are opposite to each other rather than adjacent.
The diagonals of a kite are always perpendicular to each other.
The kite shape I’m using in this piece is the one below. It looks different than the kite above but it’s still a kite. It has two pairs of equal-length sides adjacent to each other. It’s basically been squashed where the horizontal diagonal is smaller than the vertical diagonal (shown by dashed lines) as opposed to the more traditional shape, as shown in the illustration above whereby it’s the opposite; the horizontal diagonal is longer than the vertical diagonal.