Very often I need to crop to other aspect ratio, which is not defined in the list (in crop processing module), e.g. 13x18 or any other defined by me. How to do that in DT? Thanks. 
click on the crop ratio to open the section panel and enter 13:18
if you use it quite freqent then you can add plugins/darkroom/clipping/extra_aspect_ratios/13x18=13:18 to your .config/darktable/darktablerc file
(C:\Users\<username>\AppData\Local\darktable\darktablerc on windows)
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great, thanks, I was typing whithout opening the section panel!
I created a preset for this.
Anyway, the darctablerc file seems to be rewritten when DT starts …
Are you sure, it is the right place to add a new aspect ratio?
you need to edit it after you closed darktable.
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very good! so rc file is rewritten when DT is closing … great, it works now!
many thanks!