Introductory signals appear at the beginning of citation sentences. Signals are important because they indicate how a cited authority relates to the text. This relation can be supportive, comparative, or contradictory. Signals can also inform a reader what inferential degree exists between the text and cited source.
A reader can thus determine if a cited source supports or contradicts an author's assertion and whether it is necessary to take an inferential step between a cited source and the text simply by looking at the signal used.
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Bluebook Rule (20th): 1.2(a)
Signals indicating that the cited work is supportive of the author's text are the most commonly used type of signal. There are six supporting signals:
The most frequently used are probably [no signal], E.g., See, and See Also.
No signal is necessary if a cited authority:
"E.g.," is the abbreviation for the Latin phrase "exempli gratia," and can loosely be translated to mean "good example." "E.g." is used when the cited authority states the proposition and when citation to other authorities also stating the proposition would be unhelpful or unnecessary.
NOTE: "E.g." can be combined with other signals, such as "See." When communed with another signal, the other signal should be given first, separated by an italicized comma but ending with a non-italicized comma:
See is probably the most frequently used (and abused) introductory signal. It is used when the cited authority clearly supports a proposition but there is an inferential step between the proposition as stated and the cited authority. Although not a catch-all signal, it is often inappropriately used as such.
"See also" is used to cite to additional materials and authority that supports a proposition but when other authority has already been cited to using either See or [no signal]. An explanatory parenthetical stating the relevance of the additional material is strongly encouraged.
NOTE: It is not appropriate to use See also for general background-reading materials; in that case, the signal "See generally" should be used (rule 1.2(d)). Again, an explanatory parenthetical explaining the relevance of the material is strongly encouraged.
Bluebook Rule (20th): 1.2(a),(b)
Two introductory signals may be used to suggest a useful comparison: "Cf." and "Compare."
"Cf." is the abbreviation of the Latin word "confer," literally meaning "compare," but proper use of the two signals does vary in several key ways.
First, "Cf." is classified as as signal indicating support (in rule 1.2(a)) rather than one of comparison. This distinction is important when determining the order of signals and authorities, explained below.
Second, "Compare" "must be used in conjunction with 'with'" and used to offer a comparison between two or more cited authorities while "Cf." may be used to compare a single authority with the textual proposition.
Stated another way, "Cf." is used when a comparison between the textual assertion and the cited source would support the proposition by analogy, while "Compare" is used when the comparison between two or more sources will tend to support or illustrate the proposition.
When using "Compare" as a signal, "Compare," "with," and the conjunction "and" should all be italicised and both "with" and "and" should be preceded by a comma:
Compare A, with B, and C.
Once again, the use of an explanatory parenthetical with either "Cf." or "Compare" is strongly encouraged.
Bluebook Rule (20th): 1.2(c)
There are three signals for conveying negative or contradictory support:
"Contra" is used whenever the cited authority directly states a contradictory proposition and so is essentially the contradictory form of [no signal].
"But see" is used when the citation clearly supports a proposition contradictory to the textual assertion, and authority signaled by "But cf." is analogously contradictory to the textual assertion.
NOTE: There is a sort of double-negative rule for contradictory signals - if another contradictory signal has already been used, omit the "But" from either But see or But cf.
Bluebook Rule (20th): 1.3
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Bluebook rule 1.3 prescribes the appropriate order when multiple signals are used. Essentially the order of signals is supporting, comparative, contradictory, and general background. Within each general class of signal, signals are arranged in order of greatest to least direct relation to the assertion - so a signal indicating a direct quote would come before one indicating an inferential step.
NOTE: With the exception of "See generally," which would always be last if used, the order of signals as laid out in this guide is the appropriate order under rule 1.3.