Crocheting / knitting / sewing an opaque pattern

Dear users, dear authors,
What opaque technique could you use if, for example, you want to crochet, knit, or sew a shirt that doesn’t require anything underneath?

There are many different patterns that are opaque. When knitting, it’s quite simple: as long as you don’t knit a lace pattern, the project will be opaque. In crochet, too, there are many textured patterns—with or without post stitches—that are opaque. Single crochet and half double crochet stitches are usually opaque as well. I especially like linked double crochet stitches. They almost look like Tunisian crochet. By the way, Tunisian crochet usually doesn’t let anything show through either. 

Thanks for the info.
Does anyone else have anything to add?

With knitting, it’s simple, as Blickfang-Design already mentioned above. Just don’t knit any yarn overs; using knit stitches and purl stitches with the right needle size will make it opaque.

When crocheting, I find that patterns without chain stitches can turn out quite dense. It also becomes denser if you crochet multiple stitches into a single stitch—but even then, avoid chain stitches between the individual stitches. 

When it comes to crocheting, there aren’t that many options.
You can often achieve true opacity with single crochet stitches, half double crochet stitches, and perhaps even crossed stitches (i.e., crocheting the second stitch in front of the first).
You can also achieve beautiful opaque patterns using the tapestry technique, where you crochet patterns with multiple colors—and it’s sturdy, too (though it’s better suited for bags; it’s too stiff for shirts).