This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

This is a property of the original CSS Flexible Box Layout Module draft, and has been replaced by a newer standard. See flexbox for information about the current standard.

The box-lines CSS property determines whether the box may have a single or multiple lines (rows for horizontally oriented boxes, columns for vertically oriented boxes).

/* Keyword values */
box-lines: single;
box-lines: multiple;

/* Global values */
box-lines: inherit;
box-lines: initial;
box-lines: unset;

By default a horizontal box will lay out its children in a single row, and a vertical box will lay out its children in a single column. This behavior can be changed using the box-lines property. The default value is single, which means that all elements will be placed in a single row or column, and any elements that don't fit will simply be considered overflow.

If a value of multiple is specified, however, then the box is allowed to expand to multiple lines (that is, multiple rows or columns) in order to accommodate all of its children. The box must attempt to fit its children on as few lines as possible by shrinking all elements down to their minimum widths or heights if necessary.

If the children in a horizontal box still do not fit on a line after being reduced to their minimum widths, then children are moved one by one onto a new line, until the elements remaining on the previous line fit. This process can repeat to an arbitrary number of lines. If a line contains only a single element that doesn't fit, then the element should stay on that line and overflow out of the box. The later lines are placed below the earlier lines in normal direction boxes and above in reverse direction boxes. The height of a line is the height of the largest child in that line. No additional space appears between the lines apart from the margins on the largest elements in each line. For calculating the height of a line, margins with a computed value of auto should be treated as having a value of 0.

A similar process occurs for children in a vertical box. Later lines in normal direction boxes are placed to the right of earlier lines and to the left in reverse direction boxes.

Once the number of lines has been determined, elements with a computed value for box-flex other than 0 stretch as necessary in an attempt to fill the remaining space on the lines. Each line computes flexes independently, so only elements on that line are considered when evaluating box-flex and box-flex-groups. The packing of elements in a line, as specified by the box-pack property, is also computed independently for each line.

Initial valuesingle
Applies tobox elements
Computed valueas specified
Animation typediscrete
Canonical orderthe unique non-ambiguous order defined by the formal grammar


The box-lines property is specified as one of the keyword values listed below.


Box elements lay out in a single row or column.
Box elements layout in multiple rows or columns.

Formal syntax

single | multiple


Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes)-webkit ? No support (Yes)-webkit 1.1-khtml
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? ? No support ? 1.0-webkit

See also

box-flex, box-flex-group, box-pack

Document Tags and Contributors

 Last updated by: mfluehr,