# How to center the tag for a multiline diagram?

I have a diagram, say five lines, the tag (equation numbering) is in the first line, this is not I want. How to force the tag stay in the middle line? Thanks in advance!

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You could use the amsmath package, which provides an aligned environment:

\begin{aligned} \Sigma &= a + b + c + d\\ & + e + f + g + h\\ & + i + j + k + l \end{aligned}

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In the same package there's also gathered which doesn't use the & sign for alignment -- depending on your equation this may be of more use to you. –  Ant Aug 2 '11 at 9:55
And split is preferred to aligned. –  Leo Liu Aug 2 '11 at 14:35
As a I say diagram (an xymatrix), it is a one-line object occupied many lines. I want the tag is in the center line and the ","(as suggested) is in the last line. –  Ma Ming Nov 29 '11 at 22:42