Surface points aligned but it doesn't match g1

Hi everybody,
I don’t understand how to match 2 surfaces, I usually try to align control points to have a g1 continuity ( less than 0,1° difference for me) but in some case it works and in some others it doesn’t. Here is an example :

tangency.3dm (176.1 KB)

could somebody explain me the reason please ?
Thanks ! :smile:

I don’t have Rhino open right now. However from first glance, you are asking that blend surface to do a lot of work in the area around where you have changes in tangency in multiple directions.

On the upper surface, certainly to the right of the centre Isocurve, there is a lot of topological change happening. While it seems you have 3 single-span surfaces here, it may be easier to break your upper surface into either 2 surfaces, or change the form of the upper patch to relax the curvature changes, if your design intent is not affected. This will make it easier for your middle surface to match the form demands of the upper surface.

Lining up control points works best when the point spacing is the same on both sides…
tangent.3dm (83.6 KB)