I'm trying to make a bigger triangle by merging 2 smaller ones, but I get these strange overlapping corners on the top. What can I do differently to get perfect corners?
Attaching an image to illustrate this issue.
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What you are seeing are called miters. You might want to reduce your miter limit, or change your shapes' corners to rounded ones. – Vincent Mar 6 '15 at 16:35
This is a result of your stroke alignment set to center and miter joins.
In the Stroke options panel, there is a "Limit" value (short for miter limit). Setting this to 0 will remove the "strange object parts".
To add the desired sharp corners back in, add a third border triangle:
