Question

In a table I want to combine some of the columns, but not in all rows. How can I realize this with LaTeX?

Answer 1

\multicolumn{<number>}{<formatting>}{<contents>}

Where the arguments are

1. Number of columns to merge
2. the justification and formating string (just like you use in the table header, i.e. ‘|c|‘ or the like)
3. The contents to put in the merged columns

This command simply replaces the <number> columns and is used inline.

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Addition: This is also how you change the formatting of a single field in only one row of the table. Just use \multicolumn{1}{<new format>}{<contents>}.