There are few elements of punctuation that cause as much dispute among Copy Editors as when to "properly" use a hyphen. Sometimes, it seems like if you ask three different people, you'll get three different answers. For example, we all know that hyphens connect modifying words, but sometimes it is unclear when the terms modify each other.

 For the complete hyphenation rules, see the Chicago Manual of Style. For absolute accuracy, consult the latest edition of Webster's Collegiate Dictionary. Do your best, but don't worry overmuch about one or two misused hyphens; in the grand scheme of things, as long as the reader can understand what is being stated in the sentence, the information presented has served its purpose.

Using the Toolkit

The «hyphen» code will always result in a hyphen. Hyphens between numbers will always be printed as en-dashes (as in ranges). Therefore, if you want a hyphen there instead, you have to manually type in the «hyphen» code.

When to Use a Hyphen

Prefixes. Use a hyphen after the following prefixes in most words: "all-", "cross-", "ex-", and "self-.". Most "servo-" words are hyphenated (with the following two exceptions: servomechanism and servomotor).

Use a hyphen after the prefix "anti-" when it is combined with a substance or a species and modifies words such as antibody, antiserum, or any of the immunoglobulins (IgG, IgM, IgA, IgD, and IgE).

Use a hyphen after all prefixes preceding a proper noun, a number, or an abbreviation (e.g., "trans-Golgi network," "mid-1960s," or "non-GABAergic responses").

Use a hyphen if the prefix ends with "a" or "i" and the word begins with "a" or "i" (e.g., "intra-arterial" or "anti-immune").

Suffixes. Always use a hyphen with suffixes "-type," "-elect," and "-designate."

The suffix "-like" is hyphenated if 1) the root word is three or more syllables (emulsion-like), 2) a succession of three "L"s occurs with the addition of "like" (shell-like), or 3) the root word is a proper noun (an Apple-like computer).

Use a hyphen before the suffix "-fold" if the number is equal to or greater than 10 or if a specific (decimal) unit is used (e.g., 1.25-fold).  For numbers less than 10, the suffix is bumped, i.e., there is no hyphen (such as "twofold" or "ninefold").

In a combination of single-digit numbers, only the first has a hyphen (e.g., "two- and fourfold increases"); in a combination of single- and double-digit numbers in the same sentence, use numerals ("8- and 10-fold increases" or "3- and 3.5-fold increases"). Follow the same rules for ranges.

Modifiers. Some prefixes (e.g., "high-", "low-," "single," or "double-") and suffixes (e.g., "-dependent," "-independent," "-enriched" "-induced," or "-treated") receive hyphens only when they are used with a second adjective that modifies a noun. However, if this is not the case, then there is a space between the words. For example:

We performed a double-blind test; the test we used was double blind.

The test results were concentration dependent; the concentration-dependent results were recorded.

Hyphens are also used for written out ratios, for example, "analog-to-digital converter" or "length-to-width ratio."

Chemical formulas and other scientific terms. Hyphens are used in many chemical formulas to indicate the location of a specific compound, its stereo form, or separate distinct associated compounds. For example, d-arabinose, 3-chloro-4-methylbenzoic acid, F-actin, and trans-2-bromocyclopentanol are chemical formulas.

Chemicals obtained from a well-known company are sometimes designated by a model or catalog number. Use a hyphen when there are combined letters and numbers for company chemicals (for example, "SQ-19844" or "BHC-456«hyphen»3").

There are also hyphens used between amino acids in a known sequence, such as "Gly-Lys-Ala-Arg-Asp," and between the DNA positions (5′- and -3′).

In terms referring to graph locations and statistical measurement, hyphens are used before a specific, italicized variable (e.g., "x-axis," "y-intercept," or "t-test").

Sutures often have hyphens indicating the suture type/size, e.g., "6«hyphen»0 silk suture."

Hyphens are used between two ions in cotransporters; for exchangers or aniporters, use a shill ("/") between the ions.

Greek letters. Hyphens are used to separate a Greek letter before or after a term (e.g., "α-isoform" and "γ-aminobutyric acid").

Numbers and units. Hyphens are used in written out numbers and fractions. They are also used to express two parts of a whole (e.g., "95% O2-5% CO2").

Hyphenate when a number and a single unit of measure is used as a modifier or when a number and unit of measure is part of a compound adjective, e.g., "a 12-kDa fragment," "3 × 4-cm strip," or "2-μm-diameter tip."

When Not to Use a Hyphen

Prefixes. The following prefixes are not usually hyphenated: "anti-, bi-, co-, contra-, counter-, de-, extra-, infra-, inter-, intra-, micro-, mid-, multi-, non-, over-, peri-, post-, pre-, pro-, proto-, pseudo-, re-, semi-, sub-, super-, supra-, trans-, tri-, ultra-, un-, under-, and whole-."

Modifiers. Adverbs that end in the suffix "-ly" already indicate a compound description, so the hyphen is redundant (e.g., "densely packed tissue" or "genetically manipulated mice").

Chemical formulas and scientific terms. Do not use a hyphen when the numeral and unit are followed by a specific chemical compound (e.g., "165 mM MgCl2").

Hyphens are not used in Latin or foreign phrases even if they modify a noun (e.g., "post hoc testing" or "in vivo experiment").

Do not use a hyphen for specific receptors or measurements without Greek letters (e.g., "ETA receptors," "A1B adrenergic receptor," or "a wave").

Numbers. Do not hyphenate a number and unit modifier if the unit of measure has two or more parts.