The arrow that never turns around

About one person in ten writes with the hand that follows the pen across what it has just put down. In a script that runs left to right, the heel of the palm arrives at the ink before the ink has dried, and every left-hander works out some private countermeasure for it. August 13 is their day, and the smudge is the tell: the direction a writing system runs is a design decision, and somebody always pays for it.

Which makes the left arrow better company for the occasion than any hand would be. Unicode's Arrows block runs from U+2190 to U+21FF and ← is the first character in it, assigned back in Unicode 1.1, in 1993.

Text does not always run the way this sentence does. Arabic and Hebrew go right to left, and the Unicode Bidirectional Algorithm has to set both directions inside a single paragraph. Part of that work is mirroring, and rule L4 states the condition without hedging: a character is depicted by a mirrored glyph if and only if its resolved directionality is right-to-left and its Bidi_Mirrored property is Yes. Drop a left parenthesis into a right-to-left run and it is drawn as “)”. It is still the opening one — “opening” only ever meant facing the way you are reading.

The arrow is not on that list. The mirroring pairs live in a data file called BidiMirroring.txt, and no arrow appears anywhere in it. Across the whole stretch from U+2100 to U+21FF exactly one character carries the mirrored property, and it is a double-struck summation sign. Set ← inside Hebrew and it points left, exactly as it does here — which, in text running the other way, means it has quietly become an arrow pointing forward.

That is the correct call, and the reason is worth a second. A bracket is a container: its shape describes where the reader is standing, so flipping it preserves the meaning. An arrow's direction is the meaning. A ← that turned into a → inside an Arabic sentence would not be rendering the same character politely, it would be saying a different thing. Unicode declines to help, and is right to.

Direction is not only something scripts have, though — it is something you can assert. There is an invisible character for the job, the right-to-left mark, zero width and classified as plain right-to-left. You put it beside a number or a run of punctuation that the algorithm is treating as directionally neutral, and it settles which way that run leans. It has relatives: embeddings, overrides, isolates, a small family of characters whose entire function is to shape text they are never seen in.

The tidy ending here would be ↔, the arrow that refuses to pick a side. But left-handedness was never a story about symmetry. It is about being the tenth person in a system built for the other nine, where nearly everything works and one thing smears. The arrows are better company than that: they mean what they mean, and they let the sentence run whichever way it likes around them.