Apples and oranges
A comparison of apples and oranges occurs when two items or groups of items are compared that cannot be practically compared.
The idiom, comparing apples and oranges, refers to the apparent differences between items which are popularly thought to be incomparable or incommensurable, such as apples and oranges. The idiom may also be used to indicate that a false analogy has been made between two items, such as where an apple is faulted for not being a good orange.
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The idiom is not unique. In Quebec French, it may take the form comparer des pommes avec des oranges (to compare apples and oranges), while in European French the idiom says comparer des pommes et des poires (to compare apples and pears). In Latin American Spanish, it is usually comparar papas y boniatos (comparing potatoes and sweet potatoes) or comparar peras con manzanas (comparing pears and apples). In some other languages the term for orange derives from apple, suggesting not only that a direct comparison between the two is possible, but that it is implicitly present in their names. Fruit other than apples and oranges can also be compared; for example, apples and pears are compared in Danish, Dutch, German, Spanish, Swedish, Croatian, Czech, Romanian, Hungarian, Italian, Slovene, Luxembourgish, Bosnian, and Turkish. However, apples are actually more closely related to pears (both are rosaceae) than to oranges. In fact, in the Spanish-speaking world, a common idiom is sumar peras con manzanas, that is, to add pears and apples; the same thing applies in Italian and Romanian, where popular idioms are respectively sommare la mele con le pere and a aduna merele cu perele. In Czech language the idiom míchat jablka s hruškami literally means to mix apples and pears.
Some languages use completely different items, such as the Serbian Поредити бабе и жабе (comparing grandmothers and toads), or the Romanian baba şi mitraliera (the grandmother and the machine gun); vaca şi izmenele (the cow and the longjohns); or țiganul şi carioca (the gypsy and the marker), or the Welsh mor wahanol â mêl a menyn (as different as honey and butter), while some languages compare dissimilar properties of dissimilar items. For example, an equivalent Danish idiom, Hvad er højest, Rundetårn eller et tordenskrald? translates literally as What is highest, the Round Tower or a thunderclap? In Russian, the phrase сравнивать тёплое с мягким (to compare warm and soft) is used. In British English, the phrase chalk and cheese means the same thing as apples and oranges. In Argentina, a common question is ¿En qué se parecen el amor y el ojo del hacha? which translates into What do love and the eye of an axe have in common? and emphasizes dissimilarity between two subjects; in Colombia, a similar (though more rude) version is common: confundir la mierda con la pomada, literally, to confuse shit with salve. In Polish, the expression co ma piernik do wiatraka? is used, meaning What has (is) gingerbread to a windmill?. In Chinese, a phrase that has the similar meaning is 风马牛不相及 (fēng mǎ niú bù xiāng jí), literally meaning "horses and cattles won't cross into each other's area", and later used to describe things that are totally unrelated and incomparable.
A number of more exaggerated comparisons are sometimes made, in cases in which the speaker believes the two objects being compared are radically different beyond reproach. For example "oranges with orangutans", "apples with dishwashers", and so on. In English, different fruits, such as pears, plums, or lemons are sometimes substituted for oranges in this context.
Sometimes the two words sound similar, for example, Romanian merele cu perele (apples and pears) and the Hungarian szezont a fazonnal (the season with the fashion).
A Hungarian expression with a somewhat modified meaning and construction is ízlések és pofonok (smacks and slaps). It refers to the fact that you cannot decide which one is better from things that depend only on the different tastes/preferences of people, just as you cannot compare two slaps in the face if they are received by separate people.
Further, the phrase "apples-with-apples comparison" is used when an attempt is made to make sure that the comparison is fair; for example, adjusting for inflation during a discussion of wages.
Criticism of the idiom 
Various scholars have questioned the premise of the incomparable nature of apples and oranges, both in serious publications and in weblogs and spoofs (see below). These criticisms of the idiom, however, tend to assume that you cannot compare apples and oranges is a descriptive statement capable of logical or scientific counter-example, without addressing the possibility of interpreting the idiom as a normative statement (meaning something such as it's not fair to judge apples and oranges by the same criteria).
At least two tongue-in-cheek scientific studies have been conducted on the subject, each of which concluded that apples can be compared with oranges fairly easily and on a low budget and the two fruits are quite similar. The first study, conducted by Scott A. Sandford of the NASA Ames Research Center, used infrared spectroscopy to analyze both apples and oranges. The study, which was published in the Annals of Improbable Research, concluded: "[...] the comparing apples and oranges defense should no longer be considered valid. This is a somewhat startling revelation. It can be anticipated to have a dramatic effect on the strategies used in arguments and discussions in the future."
A second study, written by Stamford Hospital's surgeon-in-chief James Barone and published in the British Medical Journal, noted that the phrase apples and oranges was appearing with increasing frequency in the medical literature, with some notable articles comparing "Desflurane and propofol" and "Salmeterol and ipratropium" with "apples and oranges". The study also found that both apples and oranges were sweet, similar in size, weight, and shape, that both are grown in orchards, and both may be eaten, juiced, and so on. The only significant differences found were in terms of seeds (the study used seedless oranges), the involvement of Johnny Appleseed, and color.
The Annals rejoined that its "earlier investigation was done with more depth, more rigour, and, most importantly, more expensive equipment" than the British Medical Journal study.
Apples and oranges in teaching the use of units 
While references to comparing apples and oranges is often a rhetorical device, references to adding apples and oranges are made in the case of teaching students the proper uses of units. Here, the admonition not to "add apples and oranges" refers to the requirement that two quantities with different units may not be combined by addition, although may always be combined by multiplication, so that multiplying apples and oranges is allowed. Similarly, the concept of this distinction is often used metaphorically in elementary algebra.
The admonition is really more of a mnemonic, since in general counts of objects have no intrinsic unit and, for example, a number count of apples may be dimensionless or have dimension fruit; in either of these two cases, apples and oranges may indeed be added.
Oranges as a type of apple 
In many languages, oranges are, implicitly or explicitly, referred to as a type of apple, specifically a golden apple or a Chinese apple (confer hesperidium). For example, the Greek χρυσόμηλον (chrysomelon) and Latin pomum aurantium both literally describe oranges as golden apples. In other languages like German, Finnish, Polish, or Russian the terms for the bitter orange (a related species) are derived from Latin pomum aurantium. Additionally, the Hebrew word תפוז (tapuz) is a shortened form of תפוח זהב (tapuakh zahav), or golden apple.
In Dutch, sweet oranges are called sinaasappel, which is derived from China's apple. The Latvian apelsīns, Icelandic appelsína, Swedish apelsin, Finnish appelsiini, Russian апельсин (apelsin) and German Apfelsine share similar etymology.
See also 
- Agree to differ
- Ambiguity effect
- Apples & Oranges by Stacy Clark, an album
- Apples to Apples, a game
- Exception that proves the rule
- Genetic fallacy
- Nix v. Hedden, a U.S. Supreme Court case that partially defined "fruit" in the context of import tariffs.
- Objection to the consideration of a question
- Semantic differential
- Rhetorical device
- Law of Identity
- Umbrella term
-  "Apples-to-apples comparison"
- Scott A. Sandford, "Apples and Oranges -- A Comparison," AIR, Vol. 1, No. 3 (1995).
- Marc Abrahams, "Apples and oranges have previously been shown to be remarkably similar", Annals of Improbable Research.
- RAMAS Software, Applied Biomathematics.