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The structure of the ultimately thin crystalline allotrope of silicon oxide, prepared on a ruthenium surface, is unveiled down to the atomic scale with chemical sensitivity, owing to high resolution scanning tunneling microscopy and first principles calculations. An ordered oxygen lattice is imaged which coexists with the two-dimensional monolayer oxide. This coexistence signals a displacive transformation from an oxygen-reconstructed Ru(0001) to silicon oxide, along which laterally shifted domains form, each with equivalent and degenerate epitaxial relationships with the substrate. The unavoidable character of defects at the boundaries between these domains appeals for the development of alternative methods capable of producing single-crystalline two-dimensional oxides. [ABSTRACT FROM AUTHOR]