# Most practical way to create numbered ladder drop layout

I am searching for the most canonical way to create a skew or drop ladder equation chain layout (as described e.g. in sections 14.2.2 and 14.2.3 of the breqn package, except I don't want to use the breqn package because I like to keep control).
It seems to me this should be a no-brainer given the frequency of such layouts, yet I can't manage to find any resource about this.
My preferred solution is multline + aligned, but this does not work if one wants single line numbering:

\begin{multline}
a+b+c+d+e+f+g+h \\
\begin{aligned}
&= j + k + l + m +n\\
&= j' + k' + l'+ m' +n'\\
\end{aligned}
\end{multline}


Here are three possibilities. In the first, you alignthe = signs with another symbol of the first line, The second uses the optional argument of the \MoveEqLeft command from mathtools, and the third nests the aligned environment in a gathered environment (to fine-tune the placement of w= w.r.t. the first line, you can add to the latter some \hspace).

Unrelated: don't add a \\ at the end of a multilined environment. It produces a spurious vertical spacing, and a new equation number for an emptyline if the environment is numbered.

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{aligned} a+b+c+d+e & +f+g+h \\ &= j + k + l + m +n\\ &= j' + k' + l'+ m' +n' \end{aligned}
\medskip

\begin{aligned} \MoveEqLeft[7.2] a+b+c+d+e +f+g+h \\ &= j + k + l + m +n\\ &= j' + k' + l'+ m' +n' \end{aligned}
\medskip

\begin{gathered} a+b+c+d+e +f+g+h \qquad\\ \begin{aligned}[t] &= j + k + l + m +n\\ &= j' + k' + l'+ m' +n' \end{aligned} \end{gathered}

\end{document}


You can use TABstacks. Shown here in 3 ways, depending on the desired equation-number vertical alignment.

\documentclass{article}
\usepackage{tabstackengine}
\stackMath
\setstackgap{L}{14pt}
\begin{document}
\alignCenterstack{ a+b+c+d+e+&f+g+h \\ =& j + k + l + m +n\\ =& j' + k' + l'+ m' +n'}
\alignLongstack{ a+b+c+d+e+&f+g+h \\ =& j + k + l + m +n\\ =& j' + k' + l'+ m' +n'}
\alignLongunderstack{ a+b+c+d+e+&f+g+h \\ =& j + k + l + m +n\\ =& j' + k' + l'+ m' +n'}
\end{document}