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#### Controlling Side by Side Format

The `-y` or `--side-by-side` option selects side by side format. Because side by side output lines contain two input lines, the output is wider than usual: normally 130 print columns, which can fit onto a traditional printer line. You can set the width of the output with the `-W columns` or `--width=columns` option. The output is split into two halves of equal width, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.

The `--left-column` option prints only the left column of two common lines. The `--suppress-common-lines` option suppresses common lines entirely.