· Estimated symbol (U+212E), originally letter "e" in a particular shape but defined by the European Union as a specific symbol used in packaging to denote that a certain accuracy is guaranteed in designating volume, mass, or other quantity, · Numero sign (U+2116), used in some languages (with some glyph variation by language) to mean "number" (e.g., " 1" means much the same as "#1"). Compatibility equivalent to the letter pair "No". · Trademark sign TM (U+2122), used much the same way as the registered sign ® but about unregistered trademarks. Compatibility equivalent to the letter pair "TM" in superscript style, but glyphs vary a lot. This block does not contain all Unicode characters that have originally been formed as stylized variants of letters. Some such characters belong to other blocks due to the history of character codes. Some characters in this block are redundant duplicates of normal letters but included into Unicode for compatibility. For example, although there is a character named "kel- vin sign" in this block, it is not meant to be used instead of the normal letter "K" when expressing thermodynamic temperatures. The "kelvin sign" has been taken into Uni- code only to allow existing data to be converted to Unicode so that a distinction between normal "K" and a kelvin sign is preserved, if it exists in the original data. Thus, contrary to what many people think after finding this block, many characters in it are not more appropriate than the corresponding normal letters. It is true that using "kelvin sign" for example would contain more semantic information, since the letter "K" as such has a large number of different uses and interpretations. However, it would not be feasible to disambiguate characters by using different codes for something that is essentially identifiable as a single character. We will return to this issue in the dis- cussion of using letters in SI notations. General Punctuation The General Punctuation block (U+2000..U+206F) is very important, since many characters in it are used frequently. It is however a mixed set, and only under a very liberal interpretation can we regard all characters there as punctuation. For example, the per mille sign (U+2030) is comparable to a unit symbol rather than the comma or the colon. On the other hand, there are important punctuation characters elsewhere. The Basic Latin and Latin 1 Supplement blocks contain many very common punctuation char- acters like the comma. Moreover, characters that are used in only one script have usually been placed in the same block as the letters or other characters of the script. General Punctuation | 425