# Unwanted hard edges with subdivision modifier

This happens everytime I merge some vertices and then apply the subdivision modifier. I've tried removing double vertices and even if I delete the edge that seems to be causing the problem this happens:

It just transfers to the next edge. Does someone know why it looks like it sticks to the original mesh? Any ideas on how to fix it are welcomed!

• It doesn't sticks to original mesh, that's the subsurfed result. Likely the reason are internal faces, you could try to run Select > Select All By Trait > Internal Faces or select manually – Mr Zak Apr 11 at 9:50
• didn't work :( they are pieces that were separated and I merged them vertex by vertex (the 4 corners) and that edge was the result, I don't know if maybe that's the issue... – Inés Gallego Martín Apr 11 at 9:58
• You can provide the file using this link: blend-exchange.giantcowfilms.com – LeoNas Apr 11 at 10:57
• I can't upload it, it says that "files may take some time to process" but nothing happens – Inés Gallego Martín Apr 11 at 11:18
• I don't have any idea what's wrong exactly in your case and what were you merging to; the general principle is that Subsurf creates bulge like on the screenshot if you have: 1. internal face/-s 2. dobles. The can be created by other modifiers as well, depending on the case. In the blend-exchange.giantcowfilms.com site you're supposed to include link to the question and press Upload after choosing file, nothing is supposed to happen before that – Mr Zak Apr 11 at 16:34

Check if you have an internal quad, which is made with those edges. If there's a quad, delete it.

Also check the Mean Crease of those edges (1 means hard edge, 0 means soft edge):

• there isn't and mean crease is at 0. This edge is the result of 4 merged vertices, I get something similar everytime I do that... – Inés Gallego Martín Apr 11 at 10:44

I may be very, very late to the party but as mentioned this was more than likely due to non-manifold geometry.

In Blender 2.80 I used the advice from an answer that I now cannot find to go to select>Select all by trait and then clicked all the options until I discovered some of my verts were non-manifold. The closest approximation to the answer lies here (it gives a shortcut for selecting non-manifold geometry instead).

In the end after discovering by tweaking the settings of the aforementioned select all by trait tool that some verts were non-contiguous I asked stack again for the answer and hey presto!, just recalculate internal normals.

N.B: this is what worked for me, but tools are linked to deal with identifying your non-manifold topology issue in many scenarios