# Force vertical overlap in align

I have a multiline subscript in multiline sum which I realize via an align:

\begin{align}
P(a|b) =
&P(a,b) \\
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}


which renders as:

I would like to use the space between P(a,b) and P(b,c) so that the subscript doesn't force the net line to start any farther down than if there wasn't any subscript. I tried parbox instead of substack, but it didn't work.

I can think of two ways, either use a negative space between the first and second row, or ignore the space of the sum. I prefer the second alternative.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
P(a|b) =
&P(a,b) \\[-1.9em]
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}

\begin{align}
P(a|b) =
&P(a,b) \\
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}

\end{document}


• \smash[b]{...} just removes the depth. I'd avoid the manual adjustment: the second way is the right one. – egreg Mar 31 '17 at 9:52
• @egreg: Thanks, I have updated. I agree, the second is preferred. – StefanH Mar 31 '17 at 10:07

Assuming you don't want to number each row, you can use an aligned subblock:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\begin{align}
P(a|b) &=
\sum_{\substack{c, d, \\ e, f}}
\begin{aligned}[t]
\cdot{} &P(b,c) \\
\cdot{} &P(c,d) \\
\cdot{} &P(d,e) \\
\cdot{} &P(e,f)
\end{aligned}\\
&= x+y.
\end{align}
\end{document}


However, I would prefer to offset the following lines as follows:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\begin{align}
P(a|b) &=
\sum_{\substack{c, d, \\ e, f}}
\begin{aligned}[t]
&P(a,b) \\