And for those of you who do not wish to read the above link, the first number in a chains name is the number of eighths of an inch between rollers. 35 chain = 3/8" between rollers. 40 chain (because it's stronger than 41 and everyone seems to like 41) = 4/8" or 1/2" between rollers. And so on for larger sizes.
i didnt know the sizes, because over here we class them by, 6mm,8mm, half inch, seeing #40 is rare, and since i had no reason to ever convert the sizes i never looked into it.