\begin{definition}[Definition:Leibniz Harmonic Triangle/Diagonal]
Consider the Leibniz harmonic triangle:
{{:Definition:Leibniz Harmonic Triangle}}
The $n$th '''diagonal''' of Leibniz harmonic triangle consists of the entries in row $n + m$ and column $m$ for $m \ge 0$:
:$\left({n, 0}\right), \left({n + 1, 1}\right), \left({n + 2, 2}\right), \ldots$
Hence the '''diagonal''' leading down and to the right from $\left({0, 0}\right)$, containing  the reciprocals of the non-negative integers, is identified as the '''zeroth diagonal''', or '''diagonal $0$'''.
\end{definition}