Joining lines in Illustrator is necessary when you want to join two halves of an object into one single shape. We show you how in our Adobe Illustrator courses. 
The technique would be used to create the garment designs shown here. Firstly a photograph of the garment would be placed, ticking the Template box in order to easily draw around half of the shape. Note that two layers are created using this method – one is a locked template layer and the layer on top is for tracing the image. So, one half of the outline is drawn with either Pen or Pencil tool, then use the Reflect tool to mirror the half-garment across.
Now the two separate shapes need to be joined together at the top  and bottom to make the garment one single shape. This will make adding colours or prints easier. First zoom in on the top two anchor points to be joined. Then select both points with the Direct Selection tool (the white arrow), right-click > Average, then right-click again > Join. Average brings the two together, and Join welds them together.
Then you would create the details, possibly on a separate layer, and Reflect them in the same way, creating a mirrored image. Colours or prints are then added, along with a Colourway at the side.
Whether you’re a complete beginner or self-taught, our courses will help you gain self-confidence in your Illustrator workflow, as well as adding to your professional skill-set. See many examples of our Illustrator clients’ work on our Facebook page.
Other related Illustrator Blog Posts:
Illustrator workspace
Illustrator basics
Illustrator clipping masks
Illustrator live trace
Illustrator brushes