Quote:
If you reduce the tempo of the audio file by say 20% does Reaper still maintain the correct bar numbering automatically?
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Yes and no, depending on how you configure Reaper in "Project Setting", which allows the bars to be set by Beat or by Time. I generally use "time".

Now that I better understand what you are doing, here are the details about how this is handled in Reaper. Ya, it can be complicated. But it does work well.

https://www.youtube.com/watch?v=A4M4BoYcwPc

This issue goes beyound just setting the starting bar to what ever bar number you like. It has to do with how the project adjusts the "item" during tempo changes.

Or have I just sent us down a rabbit hole?? grin

BIAB – 2025, Reaper (current), i7-12700F Processor, 32GB DDR4-3200MHz RAM, 1TB WD Black NVMe SSD, 2TB WDC Blue SSD, 1TB WD Blue, 2 TB SK NVMe, 6 TB External, Motu Audio Express 6x6