# Alignment in equations

I want to write two simple equations one below the other, but it seems that aligned environment aligns to the right and I want to align them to the left. Here is the code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\begin{aligned}
S_1' (x_0) = S_n' (x_n) = 0 \quad \text{(clamped boundary conditions),} \quad \text{or} \\
S_1'' (x_0) = S_n'' (x_n) = 0 \quad \text{(natural boundary conditions)}
\end{aligned}
\end{equation*}

\end{document}


which produces

but I want S_1''(x_0) to appear directly below S_1'(x_0), not to be moved to the left. Do you please know the solution?

• You need to put alignment symbols, i.e. &, such that LaTeX knows how you wish to align. – user121799 Jan 1 '18 at 23:41
• since ypu've said in a comment that you are new to latex, a look at some basic documentation might help. for amsmath (which defines align), type texdoc amsmath at a command line prompt. a lot of other good (many more general) suggestions can be found at What are good learning resources for a LaTeX beginner? – barbara beeton Jan 3 '18 at 0:03

If you want to left-align the material, you need to provide alignments points via & symbols.

\documentclass{article}
\usepackage{amsmath} % for 'align*' environment

\begin{document}

\begin{align*}
&S_1' (x_0) = S_n' (x_n) = 0 \quad \text{(clamped boundary conditions), or} \\
&S_1'' (x_0) = S_n'' (x_n) = 0 \quad \text{(natural boundary conditions)}
\end{align*}

\end{document}


The alignment points are marked with an &. If there are several groups (columns of alignments), a further & is used to introduce each new group (except the first group), so that n columns of alignment require 2n-1 ampersands.

Here you can use this code for instance:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
S_1' (x_0) &= S_n' (x_n) = 0 &&\text{(clamped boundary conditions),} \quad \text{or} \\
S_1'' (x_0) &= S_n'' (x_n) = 0 && \text{(natural boundary conditions)}
\end{align*}

\end{document}


I also suggest these solutions based on alignat*, using the \ArrowBetweenLines command from mathtools (needless to load amsmath in this case), or flalign*:

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{alignat*}{3}
& & S_1' (x_0) &= S_n' (x_n) = 0 &\quad & \text{(clamped boundary conditions),} \\
\ArrowBetweenLines[\text{or}]%
& & S_1'' (x_0) &= S_n'' (x_n) = 0 & & \text{(natural boundary conditions)}
\end{alignat*}

\begin{flalign*}
& & S_1' (x_0) &= S_n' (x_n) = 0 &&\text{(clamped boundary conditions),} \\
& \text{or} & S_1'' (x_0) &= S_n'' (x_n) = 0 && \text{(natural boundary conditions)}
\end{flalign*}

\end{document}


• Three separate answers posted within 38 seconds of each other! – Mico Jan 1 '18 at 23:43
• @Mico: That' s for the New Year :o) – Bernard Jan 1 '18 at 23:54

you forgot on anchors for align (ampersands) :

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\begin{aligned}
S_1' (x_0)  & = S_n' (x_n) = 0 \quad \text{(clamped boundary conditions),} \quad \text{or} \\
S_1'' (x_0) & = S_n'' (x_n) = 0 \quad \text{(natural boundary conditions)}
\end{aligned}
\end{equation*}

\end{document}


• Thank you all very much! I did not even know there are some anchors for alignment... :-) I am new to Latex. – doremi Jan 2 '18 at 10:14