Most of the time I see the binomial coefficent explained using the variables $n$ and $k$:
$${\binom {n}{k}}={\frac {n!}{k!(n-k)!}}$$
I understand that the C stands for combinations (versus P for permutations) but why is the button labeled nCr and not nCk on most calculators?
Edit: Since this question got downvoted; the reason for asking is that understanding what and why the key is labeled that way helps me in remembering its function.
Probably from the formula $$C(n,r)=\binom{n}{r} = \frac{n!}{r!(n-r)!}$$ Which seems to be the most popular. These letters are just there, you can use different letters if you wish and different countries/schools can opt to do so. I googled "combinatorics permutations equations" and I see the nCr approach mostly, not just on calculators. Yet in school we used k too(sometimes n and m).
But, calculator manufacturers I presume are likely to stick to a single standard for ease of use. That's why you also see f, x and n on functions even though they could be any letters.