One of the most important organologists of the 20th century was Curt Sachs, who, as well as writing Real-Lexicon der Musikinstrumente (1913) and The History of Musical Instruments (1940), devised with Erich von Hornbostel the Hornbostel-Sachs scheme of instrument classification, which was first published in 1914 in the Zeitschrift für Ethnologie(Hornbostel–Sachs). This system classified musical instruments into four distinct categories: idiophones, membranophones, chordophones, and aerophones. This system of classification was updated several times by Sachs and Hornbostel and still continues to be updated periodically. One update to the system was made by Sachs in 1940 through the addition of a 5th category-electrophones, a category encompassing instruments which produce music electronically. Sachs' 1940 book, The History of Musical Instruments was meant to be a comprehensive compilation of descriptions of instruments from many cultures and their functions within their societies. The book is primarily divided into four chronological periods of instruments- early instruments, antiquity, the middle ages, and the modern occident. These periods are further subdivided into regions and then to significant time periods within those regions. Andre Schaeffner introduced a system based on state-of-matter of the sound-producing mechanism, giving rise to two top-level categories: solid (containing strings and percussion), and gas (containing woodwind and brass). With the invention of hydraulophone, the physics-based organology has been expanded to use solid, liquid, and gas, wherein the top-level category is the state-of-matter of the material that makes the sound. Reference to Kartomi's book, page 173.
Organology (from Greek : ὄργανον – organon , "instrument" and λόγος – logos , "study") is the science of musical instruments and their classification.  It embraces study of instruments' history, instruments used in different cultures, technical aspects of how instruments produce sound, and musical instrument classification . There is a degree of overlap between organology, ethnomusicology (being subsets of musicology ) and the branch of the science of acoustics devoted to musical instruments.