Why are labor, capital, and land traditionally denoted by the letters L, K, and T?
I'm only guessing (sorry) that $T$ for terre comes from the Physiocrats' work. K for capital is easiest than $c$ (for consumption). 

Btw, labor is not always $L$ but sometimes $N$ depending of if you mean labor force vs working population (or hours). And I've no ideas why $N$!