Short answers, pulled from the story.
A chord consists of two or more notes sounded simultaneously. Andrew Surmani states that when three or more notes are sounded together, the combination is called a chord.
Chords evolved out of early church organum which reflected different vocal ranges. In its earliest written form found in Musica enchiriadis around 900, organum consisted of two melodic lines moving simultaneously note against note.
The Baroque period covers the 17th and 18th centuries. During this era accompaniment of melodies with chords developed through figured bass techniques and dominant sevenths entered constant use.
A 2025 analysis examined 52 million chords across 680,000 songs finding G major and C major most commonly used accounting for 24 percent of all chords.
Roman numerals denote scale steps on which chords are built within harmonic analysis. Lower-case numerals indicate minor triads while upper-case numerals indicate major triads plus signs show diminished or augmented qualities.