Historical clue formulations

• Triad, e.g. — 2026-07-05
• Set of keys, maybe — 2019-06-06
• Musical trio, often — 2016-08-11
• Part of a guitar riff — 2013-07-08
• What a speaker may strike — 2013-06-01
• Key combination — 2010-06-24
• Ninth, e.g. — 2008-09-27
• It can't be played on a trumpet, e.g. — 2004-06-26
• Musical combo? — 2001-12-04
• C-D-G, e.g. — 2000-06-29
• Feeling — 1998-12-20
• Set of keys? — 1996-04-12