1) The Left-leaning double decrease
2) The double vertical decrease
3) A double decrease (non-directional)
4) The Right-leaning double decrease
I have a basic way that I suggest you try each double decrease, but there are several ways to do each one, and because we all knit differently, the way I suggest is...well...a suggestion only.
Here is a swatch of a lace design I am working on that will use all four of these double decreases, plus left and right leaning single decreases. I think you can see the subtle difference the choice of decrease might lend to a pattern with strong lines
You can try all four of these double decreases using my chart and directions for each one. You can also see some excellent illustrations of three of the double decreases here. The illustrations are not identical to the way I do each of these decreases, but again, they are another way to achieve the same result.
The pattern I showed above is a swatch of an exercise/scarf for a workshop I am planning. I will share the results with you in a few days. Until then, I will keep knitting in a desperate attempt to actually finish something!