Beginning with a rules change in 1806, the Senate has traditionally not restricted the total time allowed for debate. In 1917, Rule XXII was amended to allow for ending debate (invoking "cloture '') with a two - thirds majority, later reduced in 1975 to three - fifths of all senators "duly chosen and sworn '' (usually 60). Thus, although a bill might have majority support, a minority of 41 or more senators can still prevent a final vote through endless debate, effectively defeating the bill. This tactic is known as a filibuster.
when did the senate changed to 60 votes
1975