How do I create this shape?

[C_L_M]

This is a "swoosh" that appears inside a circle. I can create a "yin & yang", but this tapers...

[c.pfaffenbichler]

Have you tried using the Pen Tool? 

[C_L_M]

I have, but I don't properly understand it 😞

If anyone could give me a dummies' guide...

[Monika Gause]

Here's how to use it:

https://helpx.adobe.com/illustrator/how-to/use-pen-tool.html

You will need pratice. Don't expect good results from the first second. That's just not how it works.

[c.pfaffenbichler]

I would like to add one recommendation: Wherever possible try to set the curve points at horizontal or vertical extremes; that way you can lock the bezier handles to horizontal or vertical by pressing the shift-key. 

That might help keep the number of points down … 

[C_L_M]

I think I better  understand the tool - thank you!

You're right - it'll need quite a lot of parctice.

Is there a way to create a curve that is exactly a segment of a circle? So that it can easily fit in a circle drawn with th elipse tool.

[C_L_M]

and even more practice with my typing

[SJRiegel]

You can use a copy of the circle to crop your other shape with pathfinder. 

Another option would be to chose Draw Inside mode on your circle, and then paste the shape in, or use the circle as a clipping mask over the the shape.

[Dan Rodney]

There are many ways to do this, but one way you might want to try is the using the Width Tool (for a variable width stroke). Here's a quick video explaining the tool. https://www.youtube.com/watch?v=2yajWpxHA9A 

[Bobby Henderson]

The most precise way is to use a series of geometric sphere shapes, de-constructing and re-constructing segments of those paths to build the finished product. Draw a 6" diameter circle. Make a pair of 3" diameter circles and align them to the center left and center right sides of the larger circle. You'll have all the paths in place you need to build the ying-yang shapes. Just break apart the segments of the circles you need to build the two ying yang shapes.

[pixxxelschubser]

how to: yin yang geometric construction | Google