[atvcatole]

I believe they used Parisian variants of the french units.

They measured the distance between the Mediterranean Sea and the North Sea through Paris with platinum rulers measuring 2 Toise de Paris.

The toise was different in different parts of France, so it was specifically the Paris one they used, and the goal was to get rid of local variants of the same units with vague definitions like toise, point, line, inch, feet, mil(e) etc.

Once they had the distance in Toise de Paris and did some math they could define the circumference of the earth and define the meter at 1/40 000 000 of that.

That length was 443.44 Lignes de Paris where a ligne is 1/864 toise.