Horizontal Lines in Shape

[BenSCR]

How can I make 'Horizontal Lines' within a Triangle (or any shape) like in the attached photo...

[Monika Gause]

Draw the lines.

Draw a triangle shape on top of them

Make a clipping mask.

[Myra Ferguson]

It looks like the fill in the triangles are curved, so I'd add to Monika's instructions on step 1 to draw 2 curves and use the Blend Tool to make a blend. Then draw the triangle, place it over the blend, select both the triangle and the blend, and make the clipping mask (Object > Clipping Mask or Ctrl + 7).

The 2nd Triangle looks like it's filled with straight lines that change their angle slightly, so make 2 lines at the appropriate angles, make another triangle, place the triangle over this blend, select the triangle and the blend, and make another clipping mask.

[pixxxelschubser · Accepted solution]

At first:

The lines are not horizontal and not "lines".

I don't know in which way they was created.

But you should be able to simulate a similar drawing.
Create a long vertical rectangle (for example 0.05in x 8in) and remove the lower right anchor point. You'll get a long triangle. Use transform effect as shown in the following screenshot.

The other structure is a blend between two ellipses.

And last not least: the final result depends very much on combinations of blend modes and different opacities.

transformation effectclipping maskblendclipping maskpossible result