For a successful clipping, it is necessary to have the datasets in the same projection.

Jut rightclick on one of your layers, Save As ... and choose the CRS of the other one.

On-the-fly reprojection is just for visualization of the data, and not helpful for clipping.

Why can't I clip vectors with 2 different projections?
I would like to have the following output:


Text

(a) Text

Text
Text


That is, after a subitem, I want to return to the last number without creating a new one. Is there any way to do this without ERT?
