Arrows
Single arrows (→ ← ↑ ↓ ↔) suit notes, diagrams and headings: they show direction without weighing down a line. Double arrows (⇒ ⇐ ⇔) carry meaning in maths and logic — implication and equivalence, as in "A ⇒ B". Outside that context a double arrow reads as a claim about causality, so it is a poor decorative choice.