(howto-resume-alignment)=
# How to continue from a previous alignment

This guide shows you how to start from an alignment that was uploaded earlier, to refine it or to
compare it with another.

## Steps

1. Open the session. Each shank starts from its most recent uploaded alignment, or from a
   recovered alignment if one was saved with **Save Progress** (see {ref}`howto-recover-alignment`).

2. To start from a different one, choose it from the alignment dropdown in the top right corner. The
   alignments are listed newest first, by the date and time they were made (and the user, when one
   is known), with `original` (no alignment) last. The reference lines of the chosen alignment are
   drawn and the fit is applied straight away.

   ```{image} ../../_static/how_to/align/previous_alignment.png
   :alt: The alignment dropdown open, listing two new alignments, the 2021 alignment and original
   :align: center
   ```

3. Move, add or remove reference lines and apply the fit as usual.

4. Upload. The new alignment is added to `prev_alignments.json`; the earlier ones are kept.

With more than one shank, the dropdown applies to the active shank. Switch shanks first.

## Related

- {ref}`ref-output-files`: `prev_alignments.json`
- {ref}`explanation-how-alignment-works`: what is saved with an alignment