Subscript and superscript
A subscript or superscript is a character that is set slightly below or above the normal line of type, respectively. It is usually smaller than the rest of the text. Subscripts appear at or below the baseline, while superscripts are above. Subscripts and superscripts are often used in formulas, mathematical expressions, and specifications of chemical compounds and isotopes, but have many other uses as well.
In professional typography, subscript and superscript characters are not simply ordinary characters reduced in size; to keep them visually consistent with the rest of the font, typeface designers make them slightly heavier than a reduced-size character would be. The vertical distance that sub- or superscripted text is moved from the original baseline varies by typeface and by use.
In typesetting, such types are traditionally called "superior" and "inferior" letters, figures, etc., or just "superiors" and "inferiors". In English, most nontechnical use of superiors is archaic. Superior and inferior figures on the baseline are used for fractions and most other purposes, while lowered inferior figures are needed for chemical and mathematical subscripts.
Uses
A single typeface may contain sub- and superscript glyphs at different positions for different uses. The four most common positions are listed here. Because each position is used in different contexts, not all alphanumerics may be available in all positions. For example, subscript letters on the baseline are quite rare, and many typefaces provide only a limited number of superscripted letters. Despite these differences, all reduced-size glyphs go by the same generic terms subscript and superscript, which are synonymous with the terms inferior letter and superior letter, respectively. Most fonts that contain superscript/subscript will have predetermined size and orientation that is dependent on the design of the font.Subscripts that are dropped below the baseline
Subscripts are used in chemical formulas. For example, the chemical formula for glucose is C6H12O6. The chemical formula of the water molecule, H2O, indicates that it contains two hydrogen atoms and one oxygen atom.A subscript is also used to distinguish between different versions of a subatomic particle. Thus electron, muon, and tau neutrinos are denoted and. A particle may be distinguished by multiple subscripts, such as for the triple bottom omega particle.
Similarly, subscripts are also used frequently in mathematics to define different versions of the same variable: for example, in an equation x0 and xf might indicate the initial and final value of x, while vrocket and vobserver would stand for the velocities of a rocket and an observer. Commonly, variables with a zero in the subscript are referred to as the variable name followed by "nought".
Subscripts are often used to refer to members of a mathematical sequence or set or elements of a vector. For example, in the sequence O =, O3 refers to the third member of sequence O, which is 800.
Also in mathematics and computing, a subscript can be used to represent the radix, or base, of a written number, especially where multiple bases are used alongside each other. For example, comparing values in hexadecimal, denary, and octal one might write Chex = 12dec = 14oct.
Subscripted numbers dropped below the baseline are also used for the denominators of stacked fractions, like this:.
Subscripts that are aligned with the baseline
The only common use of these subscripts is for the denominators of diagonal fractions, like or the signs for percent %, permille ‰, and basis point ‱. Certain standard abbreviations are also composed as diagonal fractions, such as , , , or in Spanish .Superscripts that typically do not extend above the ascender line
These superscripts typically share a baseline with numerator digits, the top of which are aligned with the top of the full-height numerals of the base font; lowercase ascenders may extend above.Ordinal indicators are sometimes written as superscripts, although many English-language style guides recommend against this use. Romance languages use a similar convention, such as 1er or 2e in French, 4ª and 4º in Galician, Brazilian Portuguese and Italian, or 4.ª and 4.º in European Portuguese and Spanish.
In medieval manuscripts, many superscript as well as subscript signs were used to abbreviate text. From these developed modern diacritical marks. Also, in early Middle High German, umlauts and other modifications to pronunciation would be indicated by superscript letters placed directly above the letter they modified. Thus the modern umlaut ü was written as uͤ. Both vowels and consonants were used in this way, as in ſheͨzze and boͮsen. In modern typefaces, these letters are usually smaller than other superscripts, and their baseline is slightly above the base font's midline, making them extend no higher than a typical ordinal indicator.
Superscripts are used for the standard abbreviations for service mark and trademark. The signs for copyright © and registered trademark ® are also sometimes superscripted, depending on the typeface or house style.
On handwritten documents and signs, a monetary amount may be written with the cents value superscripted, as in $ or. Often the superscripted numbers are underlined: $,
Superscripts that typically extend above the ascender line
Both low and high superscripts can be used to indicate the presence of a footnote in a document, like this5 or this.xi Any combination of characters can be used for this purpose; in technical writing footnotes are sometimes composed of letters and numbers together, like this.A.2 The choice of low or high alignment depends on taste, but high-set footnotes tend to be more common, as they stand out more from the text.In mathematics, high superscripts are used for exponentiation to indicate that one number or variable is raised to the power of another number or variable. Thus y4 is y raised to the fourth power, 2x is 2 raised to the power of x, and the equation includes a term for the speed of light squared. This led over time to an "abuse of notation" whereby superscripts indicate iterative function composition, including derivatives. In an unrelated use, superscripts also indicate contravariant tensors in Ricci calculus.
The charges of ions and subatomic particles are also denoted by superscripts. is a negatively charged chlorine atom, is an atom of lead with a positive charge of four, is an electron, is a positron, and is an antimuon.
Atomic isotopes are written using superscripts. In symbolic form, the number of nucleons is denoted as a superscripted prefix to the chemical symbol. The letters m or f may follow the number to indicate metastable or fission isomers, as in or.
Subscripts and superscripts can also be used together to give more specific information about nuclides. For example, denotes an atom of uranium with 235 nucleons, 92 of which are protons. A chemical symbol can be completely surrounded: is a divalent cation of carbon with 14 nucleons, of which six are protons and 8 are neutrons, and there are two atoms in this chemical compound.
The numerators of stacked fractions usually use high-set superscripts, although some specially designed glyphs keep the top of the numerator aligned with the top of the full-height numerals.
Alignment examples
This image shows the four common locations for subscripts and superscripts, according to their typical uses. The typeface is Minion Pro, set in Adobe Illustrator. Note that the default superscripting algorithms of most word processors would set the "th" and "lle" too high, and the weight of all the subscript and superscript glyphs would be too light.Another minor adjustment that is often omitted by renderers is the control of the direction of movement for superscripts and subscripts, when they do not lie on the baseline. Ideally this should allow for the font, e.g. italics are slanting; most renderers adjust the position only vertically and do not also shift it horizontally. This may create a collision with surrounding letters in the same italic font size. One can see an example of such collision on the right side when rendered in HTML. To avoid this, it is often desirable to insert a small positive horizontal margin , or a negative margin before a subscript. It is more critical with glyphs from fonts in Oblique styles that are more slanted than those from fonts in Italic style, and some fonts reverse the direction of slanting, so there is no general solution except when the renderer takes into account the font metrics properties that specifies the angle of slanting,
However the same problem occurs more generally between spans of normal glyphs when slanting styles are mixed.