A horizontal, reflowable KDP book needs two different confirmations. First, verify that the EPUB records horizontal text flow and left-to-right page progression. Then verify that the text actually reflows when the reading environment changes.
This article completes both checks. It does not treat one screenshot or one comfortable font size as proof that the setting works.
Separate the internal declaration from rendered reflow
The two checks answer different questions.
- Internal declaration: What writing direction and page progression are recorded in the EPUB?
- Rendered reflow: Does the reading environment rearrange lines without losing content or order?
An EPUB can contain the intended direction values and still need a rendering check. Conversely, a file that appears acceptable in one preview does not reveal which internal values produced that result. Keep the two records separate.
What the horizontal setting writes
In the tested rune Studio workflow, writing direction is selected once in the book information. Reading the exported EPUB confirmed two results:
- Text flow was horizontal, left to right
- Page progression was left to right
The same operation test exported the comparison setting with different direction values, confirming that one setting affected both declarations. This evidence is limited to the exported file. It does not establish KDP acceptance or identical rendering on every device.
Check the EPUB before opening Previewer
Use the latest exported EPUB and record its filename. Confirm the horizontal text-flow and left-to-right page-progression declarations inside that file.
If either value is wrong, return to the writing-direction setting and export again. Do not try to correct an internal declaration by changing text size or a view option in the reader.
If both values are right, keep that result as the source-side check and continue to the rendered reflow check.
Choose three representative passages
Do not inspect only the easiest paragraph. Mark three locations before opening Kindle Previewer:
- A plain body paragraph with several wrapped lines
- A heading followed by body text
- A paragraph immediately before or after an image, list, or other non-body element used in the book
These passages reveal different failure modes. The body paragraph tests line rearrangement, the heading tests document order, and the mixed-content location tests whether surrounding content stays associated as the layout changes.
If the book does not contain images or lists, use another structural boundary, such as the end of a chapter.
Perform the reflow check in Kindle Previewer
KDP's official help lists EPUB as a supported eBook manuscript format and recommends validating it with Kindle Previewer before upload. Check the current supported eBook formats when preparing the submission.
Open the same EPUB whose internal declarations you recorded, then:
- Read the three passages at the initial text size
- Increase the text size and revisit the same passages
- Reduce it again and confirm the content sequence
- Repeat any passage that contains an image, list, or heading boundary
The purpose is not to preserve the same line breaks. In reflowable content, changing the available space is expected to change line wrapping.
Use an explicit pass/fail rule
Pass the reflow check when all of the following remain true after the text-size change:
- The complete text is still present
- Reading order is unchanged
- Headings remain attached to the intended following content
- Images or other elements remain in the intended position relative to the prose
- Text does not overlap, disappear, or become clipped
Fail the check when content is missing, reordered, obscured, or detached from its intended context. A different number of lines is not a failure by itself; it is the expected result of reflow.
This criterion makes the article's promise reproducible: internal direction values pass at the file layer, while preserved content and order across text sizes pass at the rendering layer.
Keep the two results in the build record
Record:
- EPUB filename
- Internal text flow: horizontal
- Internal page progression: left to right
- Previewer passages checked
- Text sizes compared
- Reflow decision and any failed location
If you rebuild after a correction, create a new record for the new EPUB. Do not reuse a pass recorded against an older file.
Who this workflow suits
This workflow suits reflowable novels, nonfiction, manuals, and other text-led eBooks for English-language readers. It does not determine whether a title should use fixed layout; that is a separate publishing decision.
Scope note: rune Studio evidence here covers the declarations in the exported EPUB and the ability to inspect the finished file. Kindle conversion, store acceptance, and per-device rendering remain separate checks.
Summary
- Confirm horizontal text flow and left-to-right page progression inside the EPUB
- Use Kindle Previewer to compare the same passages at more than one text size
- Expect line breaks to change; require content, order, and relationships to remain intact
- Record both the internal result and the rendered reflow result against the exact EPUB filename
Export one current build, mark three representative passages, and complete both layers now. The horizontal setting is confirmed only when the internal declarations and the reflow decision are recorded for the same file.
See the current product scope on the Rune Studio product page.
