Combining different yarn weights

Dear community,
What should you keep in mind when combining different yarn weights in a craft project?

It really depends on what you’re trying to achieve. Do you want it to look intentionally different, like Ina’s awesome scarf, or do you just have different yarns on hand?
If you want it to look even, the only way is to do a gauge swatch or use the thinner yarn doubled, or something like that. I’m currently making a patchwork blanket out of various scraps, so sometimes I’ve knitted with 2 strands and sometimes with 3, or—after checking the gauge—if the differences are minor, I’ve added or subtracted a few stitches and rows to ensure the squares end up roughly the same size.

Here’s another option.

To use up my smaller scraps, I crocheted together various thicknesses and materials—I always worked with different numbers of strands at a time, so that the final yarn thickness was always similar. This works really well for tightly crocheted pieces like this.

You can also use different stitch patterns to even out the width. I once knitted a sweater with wide stripes using glittery bobble yarn and smooth ribbon yarn. I knitted the bobble yarn in stockinette stitch and the ribbon yarn in brioche stitch. Brioche stitch naturally expands widthwise.