dcolumn

dcolumn is a LaTeX package that provides decimal alignment in tabular and array environments. Its main purpose is to make numbers align on the decimal point across multiple rows, improving readability in tables that mix numeric values with varying widths.

The package introduces a new column type, typically denoted as D, which allows the user to specify how numbers should be treated for alignment. A D column splits each entry at the decimal marker into an integer part and a fractional part, and then typesets the parts so that the decimal points line up vertically with other D columns in the same table. One of the design goals is to allow numbers with different decimal separators (for example, a dot or a comma) to be aligned consistently, by providing separate input and output decimal markers.

Typical usage involves loading the package with \usepackage{dcolumn} and using the D column specifier in the tabular preamble, for example in a tabular environment such as l D{.}{.}{-1} r. The exact third argument controls the number of decimal places to reserve or how the alignment should be determined; negative values commonly indicate automatic calculation of the necessary width. The effect is that entries like 12.3, 4.56, and 123 align on the decimal point within the same column.

dcolumn is an older but still useful tool for simple decimal alignment. It is often contrasted with more modern packages such as siunitx, which offers a broader set of numerical formatting features. For complex or highly customized numeric formatting, newer packages may provide additional options, but dcolumn remains a lightweight solution for straightforward decimal alignment in LaTeX tables.