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The CSS block flow direction is the direction in which block-level boxes and successive line boxes progress in the relevant flow. The writing-mode property determines this direction: horizontal-tb progresses top to bottom, vertical-rl right to left, and vertical-lr left to right.^[ref]

For an own geometry example, take three ordinary in-flow block boxes with zero margins, borders and padding, each having inline size $100$ and block size $30$ CSS pixels. Assume no conflicting sizing constraints and a containing region sized to fit them. In horizontal-tb, the parent is $100$ pixels wide and $90$ high. The boxes occupy successive vertical intervals $[0,30]$, $[30,60]$ and $[60,90]$.

In vertical-rl, those same CSS logical dimensions give a parent $90$ pixels wide and $100$ high. Measuring physical $x$ from the left edge, the first box occupies $[60,90]$, the second $[30,60]$ and the third $[0,30]$. Their source order is unchanged, but the block progression runs toward decreasing physical $x$. These are stipulated box geometries, not font-dependent text measurements.^[ref]

CSS inline direction concerns progression within a line and its base directionality. Setting direction: rtl in an otherwise horizontal example does not make the block stack run upward: writing-mode still supplies its top-to-bottom block flow. Likewise, the rl in vertical-rl describes block progression rather than simply naming right-to-left text ordering.

This distinction also affects CSS scroll origin, whose default anchor uses block-start and inline-start. In the horizontal example, block-start is top; in the vertical right-to-left block flow it is right. Rules expressed using logical starts and sizes can therefore follow the layout convention instead of hard-coding one physical edge.^[ref]

A writing-mode change determines line orientation and block progression, while CSS glyph orientation determines how text is laid out inside those lines. Treating all three as a single rotation would lose these separate layout effects.

A CSS logical dimension measures a box relative to text flow rather than a fixed physical axis. The inline dimension is parallel to the flow within a line; the block dimension is perpendicular to it. CSS inline size and CSS block size are measurements along those respective dimensions.^[ref]

Let the inline size be $I$ and block size $B$. In a horizontal writing mode, physical width and height areIn a vertical writing mode, their roles exchange:These equations describe the dimension mapping; the box's actual used sizes still follow the applicable sizing and layout rules.^[ref]

For a simple definite-size example, take inline size $180$ and block size $60$ CSS pixels, with no padding, borders or conflicting constraints. The physical box is $180$ by $60$ in horizontal-tb, but $60$ by $180$ in vertical-rl or vertical-lr. The product $IB=10800$ is unchanged in this stipulated rectangle even though the physical width differs.

A direction reversal is a different operation. Changing CSS inline direction can exchange inline-start and inline-end while leaving the dimension itself the same. Likewise, vertical-rl and vertical-lr have the same inline/block axis mapping but different CSS block flow directions. An axis, its size and its directed start edge are related quantities with separate meanings.^[ref]

Logical dimensions help describe reusable layouts in terms such as the length available for a line and the extent available for successive lines or blocks. The specification's terms logical width and logical height refer to inline and block size respectively; they should not be read as promises about physical width and height in every mode.

This mapping also matters when a child uses a different writing mode from its containing context. Determine which box's CSS writing mode supplies each sizing reference before treating a number as a physical width. The dimension definitions provide the vocabulary, while the rules for the particular layout resolve its constraints.