Alternate & Mix: When the Backs Come Out Reversed
Alternate and mix is the repair for two-pass scanning, where all the fronts and all the backs arrive as separate files. Most bad results are not a tool problem but a source-order problem, and the first few pages tell you which.
Why this comes up
A single-sided scanner cannot see the back of a sheet, so duplex documents are captured in two passes. The backs come off the second pass in reverse order, and any page lost in either pass shifts every later pair.
Check before you run it
- Compare the page count in both files. Each sheet should contribute one front and one back. If the counts differ, find the missing page before mixing; otherwise every pair after it is misaligned.
- Reverse the backs file when it was scanned from the bottom. A second pass that starts with the last sheet in the stack puts the backs in reverse. Set the step to 2 and reverse that document, then mix.
- Preview the first four pages, not the last four. The opening pages expose an order mistake immediately: page two should be the back of page one. If it is another front, the reverse setting is wrong.
When to reach for it
- A two-pass scan is about to go to a client. Verify the seams at the start, the middle and the end before sending; a single missing back is easy to spot in a preview and expensive to explain later.
- The backs file came back in reverse order. Reversing that file before mixing restores the reading order without rescanning the stack.
One question we get
What is Alternate & Mix for?
The classic two-pass scanning trick: scan all fronts into one PDF and all backs into another, then let the tool interleave them back into reading order. Each file can have its own step and reversed order.
Where it fits
The tools people usually open next to Alternate & Mix:
- Split in half — Split two page layout scans, A3 to double A4 or A4 to double A5.
- Merge PDF — Combine multiple PDFs and images into one document.
- Extract Pages — Get a new document containing only the desired pages.
Every tool works on a copy, so the original file is never touched. Uploads are stored under an unguessable id and deleted automatically two hours after you last open them; nothing is listed, shared or indexed. More on the security page.