The Letterlike Symbols block — U+2100 through U+214F, 80 codepoints — holds the curious in-between of Unicode: characters that look like letters of the Latin or Greek or Hebrew alphabets, but whose meaning is something more specific than the letter itself. ℝ is not just a styled R; it is the mathematical sign for the set of real numbers. ℓ is not a fancy lowercase L; it is the litre symbol when distinguished from L, and a vector quantity in physics. The block exists because typographic distinctions like these carry semantic weight that plain Latin letters cannot.
About this block
The block was added in Unicode 1.0 (1991) with most of its contents arriving in the early versions and small additions continuing through Unicode 5.2. Its inhabitants fall into a few clear families. The double-struck (blackboard-bold) letters used in mathematics for canonical number sets are the most famous: ℂ U+2102 COMPLEX, ℍ U+210D QUATERNIONS, ℕ U+2115 NATURALS, ℙ U+2119 PRIMES (or projective space), ℚ U+211A RATIONALS, ℝ U+211D REALS, and ℤ U+2124 INTEGERS. The convention of using blackboard bold for these number systems began in lecture halls during the mid-twentieth century — chalk on blackboards cannot produce true boldface, so mathematicians began doubling the vertical strokes of capitals to imitate the bold typeface used in printed textbooks. The style stuck. Other double-struck letters like ⅅ U+2145 and ⅆ U+2146 are used for the differential operator d in differential geometry.
Several Hebrew letters appear here because they were borrowed wholesale into mathematics. ℵ U+2135 ALEF SYMBOL is Georg Cantor's notation for transfinite cardinal numbers: ℵ₀ is the cardinality of the natural numbers, ℵ₁ the next infinite cardinal, and so on. ℶ U+2136 BET SYMBOL, ℷ U+2137 GIMEL SYMBOL, and ℸ U+2138 DALET SYMBOL extend the same idea. These are mathematical symbols, not Hebrew letters; they have no joining behaviour, no RTL property, and are governed by a Letter_Other Unicode category rather than the script-specific Hebrew categorisation.
The block also holds the working abbreviations of commerce and units. ™ U+2122 TRADE MARK SIGN and ℠ U+2120 SERVICE MARK SIGN are the unregistered-trademark superscripts (the registered ® lives in the Latin-1 Supplement block at U+00AE). ℗ U+2117 SOUND RECORDING COPYRIGHT is the phonogram copyright sign required on commercial audio recordings. ℅ U+2105 CARE OF is the postal abbreviation; № U+2116 NUMERO SIGN is the European and Cyrillic "Number" abbreviation; ℡ U+2121 TELEPHONE SIGN is the "TEL" abbreviation that decomposes under NFKD to the three letters T-E-L. ℞ U+211E PRESCRIPTION TAKE is the apothecary's Rx mark.
The two temperature-unit signs deserve special attention because they expose a Unicode quirk. ℃ U+2103 DEGREE CELSIUS and ℉ U+2109 DEGREE FAHRENHEIT are encoded as singleton precomposed characters, but their NFKD compatibility decompositions break them apart into U+00B0 ° + U+0043 C and U+00B0 ° + U+0046 F respectively. This means that any text that has been normalised with NFKC or NFKD will see the precomposed ℃ replaced by the two-character sequence °C. The behaviour is intentional — Unicode discourages new use of the precomposed forms — but it can produce surprising round-trip differences if you compare a string against itself after normalisation. The dedicated normalization guide covers the rules in detail. Similar compatibility decompositions apply to ℡ → TEL, № → No, and several other Letterlike symbols. The Ohm sign Ω U+2126 decomposes to the Greek letter Omega U+03A9 — the same shape, the same name, but a different codepoint preserved only for round-trip compatibility with legacy standards.
Two more clusters deserve a mention. The Fraktur-style letters ℋ U+210B, ℌ U+210C, ℑ U+2111, ℜ U+211C, ℨ U+2128 are used in complex analysis: ℑ is the imaginary part, ℜ the real part. Script letters ℓ U+2113 SCRIPT SMALL L, ℒ U+2112 SCRIPT CAPITAL L, ℛ U+211B SCRIPT CAPITAL R serve similar mathematical-typography roles. The Weierstrass elliptic function takes its own letter ℘ U+2118 SCRIPT CAPITAL P. ℎ U+210E PLANCK CONSTANT and ℏ U+210F PLANCK CONSTANT OVER TWO PI give physics its characteristic Planck-h and reduced-Planck-h symbols at dedicated codepoints distinct from a plain italic h.