Different experts define Modern differently. For some it’s an architectural style that existed back in the day and is no more. For others (myself included), it’s a more general movement that remains viable and active today and continues to evolve with new materials, new technology, and new living needs. The earliest innovators and adopters tended to rigidly define the new architecture with rigorously defended manifestos (at least until the manifestos became inconvenient and subsequently discarded, oftentimes by the manifesto writers themselves).
There are also various offshoots of Modern, including hybrids (Streamline Moderne, a fusion of Art Deco and Modern), Functionalism (supposedly no consideration of aesthetics), Googie (Modern gone wild), and the International Style. This last “offshoot” was conjured by Henry-Russell Hitchcock in his 1927 book, Modern Architecture: Romanticism and Reintegration and popularized after the 1932 Modern Architecture show at the Museum of Modern Art in New York. Hitchcock noted that it really didn’t matter where a Modern building of that time was designed and built, it tended to look the same (flat roofs, flat surfaces, clean lines). Ironically, because photography back in those days was black and white, International Style also became synonymous with white structures, even though many of the buildings mentioned and exhibited in the 1932 show were not white. Many take the position that if a building is simply not white, it is not Modern. This creates endless entertainment when folks argue over whether a building is Modern or Contemporary.
Based on my readings, observations, and opinions, I’ve boiled Modern down to ten commandments: (1) lack of ornamentation, (2) form follows function, (3) honesty in materials and design, (4) embracing the environment, (5) eager adoption of new materials and technology, (6) a place for everything and everything in its place, (7) a focus on the human condition, (8) less is more, (9) use of the machine aesthetic, and (10) rules are meant to be broken.