Arrow Symbols

A double arrow is three different characters

Three unrelated shapes answer to it, and the names do not always say which is which.

A head at each end. U+2194 LEFT RIGHT ARROW and U+2195 UP DOWN ARROW are one shaft with two heads.
This is the sense of “double sided arrow” that matches the shape. The diagonals ↖,
↗, ↘ and ↙ have no two-headed partner here; Unicode puts U+2921 NORTH WEST AND SOUTH EAST ARROW and
U+2922 NORTH EAST AND SOUTH WEST ARROW in a block this set does not reach.

A double-stroke shaft. U+21D2 RIGHTWARDS DOUBLE ARROW is one head on a shaft drawn as two parallel
lines. Its family runs ⇐, ⇑, ⇓, ⇕ and the four diagonals, with
U+21D4 LEFT RIGHT DOUBLE ARROW carrying two heads and a double shaft at once. U+21DA LEFTWARDS TRIPLE ARROW and U+21DB RIGHTWARDS TRIPLE ARROW take the
same idea to three strokes.

Two whole arrows. U+21C4 RIGHTWARDS ARROW OVER LEFTWARDS ARROW, U+21C6 LEFTWARDS ARROW OVER RIGHTWARDS ARROW, U+21C5 UPWARDS ARROW LEFTWARDS OF DOWNWARDS ARROW and U+21F5 DOWNWARDS ARROW LEFTWARDS OF UPWARDS ARROW are two
complete arrows stacked or side by side, pointing opposite ways — a pair, not a single arrow with
something added.

TWO HEADED does not mean one head at each end

U+219E LEFTWARDS TWO HEADED ARROW, U+21A0 RIGHTWARDS TWO HEADED ARROW, U+219F UPWARDS TWO HEADED ARROW and U+21A1 DOWNWARDS TWO HEADED ARROW have both heads at the same end, one
behind the other. The name reads like a description of ↔ and is not one: none of these four
points both ways.

27 of the 140 are maths, and it shows

U+2190, U+2191, U+2192, U+2193 and U+2194 are general category Sm, maths
symbols. U+2195 — the same shape turned vertical — is not. The pattern repeats in the double
arrows: U+21D2 and U+21D4 are maths, while U+21D0, U+21D1, U+21D3 and
U+21D5 are ordinary symbols. Anything selecting by category catches → and misses
↕, and a maths layout engine may space one as an operator and not the other. The
asymmetry is in the standard, not the font — the same boundary that sorts the
math symbols.

Only 15 of them change width

125 are Neutral width and stay narrow. The 15 that are Ambiguous include all
ten compass arrows U+2190 to U+2199, plus U+21D2, U+21D4, U+21E7, U+21B8
and U+21B9 — the plainest arrows, the ones reached for first. Ambiguous means the cell count
depends on the font context, so a monospace diagram built from → holds in one context and
shifts in another; a Neutral arrow such as ⇢ avoids the question.

The display arrows are a different kind of character

35 of them come from the Dingbats run, drawn as pictures rather than marks read inside a
line: U+27A1 BLACK RIGHTWARDS ARROW, U+279E HEAVY TRIANGLE-HEADED RIGHTWARDS ARROW, U+27B5 BLACK-FEATHERED RIGHTWARDS ARROW, U+27BB HEAVY TEARDROP-SHANKED RIGHTWARDS ARROW and their relatives, with
heads, fletching, shadows and tails the plain arrows lack. All are ordinary symbols, all Neutral
width. U+2B05 LEFTWARDS BLACK ARROW is their leftward counterpart, in another block, its name
putting the same words in a different order.

WHITE is Unicode’s word for outlined: ⇦, ⇧, ⇨ and ⇩, the
pedestal forms ⇫, ⇬ and ⇭, and shaded variants such as U+27AB.
Turning arrows are U+21BA ANTICLOCKWISE OPEN CIRCLE ARROW and U+21BB CLOCKWISE OPEN CIRCLE ARROW. The rest of this
reference is at symbols to copy and paste, and arrows that
have to meet ruled lines belong with line and box symbols.