When merging lines with ctrl+j why does the intersection of lines create jagged edges?

[josh32498587eyzg]

Hi I am trying to make shapes, pretty simple ones, why is this the outcome of merging my lines (which all intersect at the vertex)

[josh32498587eyzg]

Oh, it gets worse.

[Larry G. Schneider]

Try using a round corner junction. In your first image you might try lowering the miter limit to 3 or 4 and see if you like it better. Corners are limited to 2 paths not 3.