Been meaning to write up how I use symbols more fully, but here’s the list of options I’ve churned up and played with. I only use 4-5 of these. Most of them are used in mathematics, so again, if you’re a regular user of math symbols this might not be so helpful.
I tend to try to use symbols that are easily recognized in the typeface/font size I’m using (some of these are barely visible without a large font size) and that meaningfully correspond—at least a little—with whatever they indicate.
E.g., I use △ to indicate an open project, ▲ to indicate a closed project. This comes from David Allen’s definition of work (any change we want to make happen in our worlds) and project (a project is whenever more than one action is required to complete some work).
⊗ := the tensor product, a generalization of nonlinear vector space. See https://www.math3ma.com/blog/the-tensor-product-demystified
▲ := delta, change
⋈ := bowtie, natural join of two relations
× := multiplication; cross product
∧ := logical and
∨ := logical or
∪ := Union
∩ := Intersection
∑ := summation
↯ := contradiction
≀ := wreath product
⨳ := smash product
∎ := denotes the end of a proof, aligns with the “finished” nature of these notes.
∴ := therefore; it is used at the end of articles to indicate the reader has reached the end.
∵ := because
⫝ := the forking symbol (looks like an anchor)
︎: duelling swords
︎ : hammer and chisel
☍ := the astronomical symbol for opposition
☌ := astronomical symbol for conjunction
︎ : an anchor
⎊ : inverted triangle in a circle
︎ : an atom
Others: ⟐ ⟠ ⧉ ❖ ◇ ◆ ⟁ ☉
Edit: as you can see, some of the graphical ones near the end fight with Discourse’s rendering. It seems to convert them to emoji, with some graphical glitches in tow. Be sure to test out any characters you’re thinking about using in all of the contexts you’ll use them!