What triangles mean
Triangles point, so they often work as small arrows. ▲ and ▼ mean up and down: a price change (▲ 3%), a sort order in a table, or a collapsible section (▸ closed, ▾ open). ▶︎ is the universal play button, ◀︎ goes back, and ⏩︎ ⏪︎ fast-forward and rewind. The warning sign ⚠︎ is a triangle with an exclamation mark, used for cautions and alerts.
In geometry, △ABC names a triangle and ⊿ (right triangle) marks one with a 90° angle. Old alchemy used triangles for the four elements: 🜂 fire points up, 🜄 water points down, and the same shapes with a bar across are 🜁 air and 🜃 earth. These alchemical characters are rare, so some devices show them as boxes.
Triangle or delta?
Three characters look almost the same: △ is the white up-pointing triangle (a shape), Δ is the Greek capital letter delta, and ∆ is the increment sign used in math. Use △ for shapes and bullets, and Δ when you mean "change in", as in Δt. Each has its own meaning, so search engines and screen readers treat them differently. See the delta symbol page for the Greek letter.
Within the triangles, "black" means filled and "white" means outline, and the small versions ▴ ▸ ▾ ◂ sit nicely inside text. ► and ◄ are slightly longer "pointers", while ⏵ ⏴ ⏶ ⏷ are medium-sized solid triangles.
How to type triangle symbols
On Windows, hold Alt and type a number on the number pad: Alt + 30 ▲, Alt + 31 ▼, Alt + 16 ► and Alt + 17 ◄. On a Mac, open the Character Viewer with Control + Command + Space and search "triangle". On phones, search "triangle" in the emoji keyboard for 🔺 🔻 🔼 🔽.
In Microsoft Word, type 25B2 and press Alt + X for ▲. In HTML, use ▲ for ▲ and ▼ for ▼. In LaTeX, \triangle gives △, and the amssymb package adds \blacktriangle and \blacktriangledown. For triangle-headed arrows, see the arrow symbols.