Why a 2/3 vote to protect the minority?

[Drake Savory]

My question deals more with the history of the 2/3 vote.  I don't want to get into protecting the right of the minority with a 2/3 vote such as the right to debate with Previous Question or voting to Suspend the Rules.  I'm curious where the 2/3 as the itself came from.  Is it a holdover from the British Parliament, its reason lost in antiquity?  Dis someone very mathematically point out that 2/3 means the minority is outnumbered 2 to 1?  Was it the next progression in fractions after majority of 1/2?  2/3, 3/4, 4/5, etc (except there are no 3/4 or 4/5 votes in Robert's Rules).

So how was 2/3 chosen as the higher threshold beyond a majority?

[Rob Elsman]

In "modern" times, the two-thirds vote was established in 1179 as necessary for completion of a papal election. I suspect that the use of the two-thirds vote in the common parliamentary law derives from this.

[Rob Elsman]

See the first chapter of the decree "LICET DE EVITANDA DISCORDIA" of Pope Alexander III at the Third Lateran Ecumenical Council.

[Al Dunbar]

I expect that the two thirds rule was selected originally because, while a simple majority could mean that a decision could be taken in a situation where the number of nays was very close to the number of yeas, two thirds means that there would be at least two yeas for every nay, which would be a more significant kind of majority.

Well then, why not three-quarters, four fifths, five sixths, and on and on, one might ask. I think because going down that path would lead to requiring unanimity if one could not draw the line somewhere "reasonable". It might also seem to question the good faith one should assume in a deliberative assembly.

Finally, it is very well established in practice to the extent that I do not believe it has ever been seriously challenged. So, if it works, why worry about where it came from?