In current western music working with chords or harmonies is one of the key characteristics. During the centuries different approaches were used. The meaning of the term harmony goes back to the Greek harmonia, which had in ancient times primarily mathematical and philosophical connotations. From the seventeenth century until the present day the so called "triad-based music" has been dominant. The theory of harmony developed into a separate theoretical-practical discipline. The most important coding systems which evolved during the last centuries are:

• figured bass/thoroughbass
• degrees (roman numerals)
• coding of harmonic functions (in the german theoretical tradition)
• chord symbols

Concepts (Grove): 

• chord
• chordal inversion
• dissonance
• (constructional technique and note relationships)
• tonality
• key

Article Grove Music Online: https://doi.org/10.1093/gmo/9781561592630.article.50818