One degree sign, and the marks that are not it

° U+00B0 DEGREE SIGN, category So, is the character for degrees of temperature and degrees of arc. ′ U+2032 PRIME and ″ U+2033 DOUBLE PRIME are category Po — punctuation, not symbols — and mark minutes and seconds of arc, or feet and inches. A coordinate written 51° 30′ 26″ uses all three, and none of them is the ASCII apostrophe or the ASCII double quote, which is why a coordinate typed on a keyboard and one pasted from a reference rarely compare equal.

The double prime is not a stable character

NFKC rewrites ″ U+2033 into two copies of ′ U+2032. ‴ U+2034 TRIPLE PRIME becomes three. The reversed forms ‶ and ‷ do the same to reversed primes. Afterwards the text looks identical and the string is longer: one character in, two out. A character limit, a fixed-width column, or an exact-match comparison all notice that; a reader does not. If a value has to match something stored earlier, decide which form is canonical before it is stored, not after.

℃ and ℉ are single characters, until something normalises them

℃ U+2103 DEGREE CELSIUS and ℉ U+2109 DEGREE FAHRENHEIT are one character each, category So. NFKC decomposes ℃ into two characters, U+00B0 followed by U+0043 — the degree sign and an ordinary capital C — and ℉ into U+00B0 and U+0046. So the choice between pasting ℃ and writing °C is only a real choice somewhere that leaves text alone. Where normalisation happens, the two converge, and the single character is the one that changes underneath you. In a field that normalises, the two-character form is the one that will still be there when you read it back.

Only one of these survives a latin-1 channel

U+00B0 sits below U+0100, so latin-1 and cp1252 carry the degree sign as a single byte. ℃, ℉, ′ and ″ are all above that range and have no representation in either encoding. If a value passes through something genuinely latin-1 — an old export, a mail header, a legacy column — the degree sign is the one that arrives, and the primes and the precomposed unit signs are the ones that turn into a substitution character or vanish.

Three characters that look like a degree sign and are not

U+00BA MASCULINE ORDINAL INDICATOR is a letter, category Lo, used in Spanish and Portuguese ordinals; NFKC turns it into a plain lowercase o. U+02DA RING ABOVE is a modifier letter, category Sk, meant to sit over another letter. U+2070 SUPERSCRIPT ZERO is a raised nought. Each is drawn as a small raised circle, none of them is U+00B0, and a unit parser or a search looking for the degree sign matches none of them.

Width

°, ′, ″, ℃ and ℉ have East Asian width Ambiguous; ‴, ‶ and ‷ are Neutral. So in a monospace column rendered with ambiguous characters wide, a double prime and a triple prime are not the same width, and a table of coordinates padded out with spaces stops lining up.

The wing index is the maths and science symbols list. The raised digits that sit beside a degree sign in a formula are on exponent symbols, and the operators are on math symbols.