# Why are Legendre polynomials normalised as P(1) = 1?

I understand that standard Legendre polynomials are normalised such that $$\int_{-1}^1{P_l(x) \cdot P_m(x) ~ dx} = \frac{2}{2l + 1} \delta_{lm}.$$What is the historical reason for this? Does it simplify anything? Who decided on this convention?

Does it simplify anything? Yes, for example $$P_{n}(x)={\frac {1}{2^{n}n!}}{\frac {\mathrm {d} ^{n}}{\mathrm {d} x^{n}}}\left(x^{2}-1\right)^{n}$$ and $${\frac {1}{\sqrt {1-2xt+t^{2}}}}=\sum _{n=0}^{\infty }P_{n}(x)t^{n}$$ See Wikipedia link.
Answering your original question with some edit (by myself), this is not a historical issue, it treats only of the usual procedure for normalisation of any set of orthogonal functions. I mean, in mathematics and statistics, given any set of $$n$$ L.I. functions $$f_k(x)$$, $$k=1,2,\ldots,n$$, with which we are intending to form a basis (for the corresponding $$n$$-dimensional vector-space), the orthonormality condition is just that $$\int_a^b{f_m(x) \cdot f_n(x) ~ dx} = \delta_{mn}$$.