3213: Dental Formulas
| Dental Formulas |
 Title text: I mean, half of these are undefined. And your multiplication dots are too low; they look like decimal points. |
Explanation
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This is one of 70 incomplete explanations: This page was created recently. Don't remove this notice too soon. If you can fix this issue, edit the page! |
A dental formula specifies the number of teeth of each type on each side of the jaw, with dots separating the numbers. There are two rows, representing the upper and lower jaw, separated by a horizontal line. The number of incisors is indicated first, canines second, premolars third, and finally molars, so the formula in the comic would represent 3 incisors, 1 canine, 3 premolars, and 1 molar on each side of the upper jaw, and equal numbers in the lower jaw except only 2 premolars. This is the dental formula for the cat family. The adult human dental formula is 2.1.2.3 for both the upper and lower jaw.
Cueball is (wrongly) treating a dental formula as an arithmetic expression, with the line indicating division and the dots indicating multiplication. In the title text he notices that "half the formulae are undefined" representing animals that lack one of the four types of teeth listed above. He also notes that the "dots are too low", as in fact the dots in a dental formula are at the height of period characters and aren't meant to imply multiplication.
'Mammologists' is also misused; the correct term is mammalogists. Mammologists study human breasts specifically, usually in a medical context.
Transcript
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This is one of 46 incomplete transcripts: Don't remove this notice too soon. If you can fix this issue, edit the page! |
- [Cueball and Megan are standing in front of a whiteboard, on which is written
- 3.1.3.1
3.1.2.1
- 3.1.3.1
- along with some other scribbles.
- Underneath the panel is the caption, "Mathematicians encounter dental formulas".]
Cueball: Do mammologists think these are hard?
Cueball: I mean this one just evaluates to 3/2.
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First!AmethystSky14 (talk) 21:43, 27 February 2026 (UTC)
The top left drawing is a tooth. Xkdvd (talk) 22:04, 27 February 2026 (UTC)
This confused me for a long time (partly due to the mammal/mammol thing) - I took them to be dentists. I'm now inferring that the counts are typical of a species rather than descriptive of an individual patient. Maybe the write up could make that more clear in case someone else as dumb as me passes by 2A00:23EE:10C8:110F:D992:D45:1C7A:DF02 guest
- "in case someone as dumb as me passes by" - that would be everyone, see Explain xkcd: It's 'cause you're dumb. 64.201.132.210 22:21, 27 February 2026 (UTC)
- Yes, dental formulas are based on typical, not individual, dentition. In cases where it frequently varies (like humans with their unreliable wisdom teeth) you sometimes see a range. 70.40.90.209 02:29, 28 February 2026 (UTC)
Because I'm sure someone else will be wondering, based on a very cursory search, the formula on the board appears to be permanent teeth for felines. At the very least, Wikipedia's entry on Dentition lists this formula for cats, lions, and tigers. Perhaps an actual expert will come along and shed further light on this. 97.116.61.145 22:52, 27 February 2026 (UTC)
Some British sources still use a baseline dot (full stop/period) as the multiplication symbol and a midline dot (interpunct) as the decimal point. These sources could write 3.2·1 = 6·3. Scary. Even The Lancet uses the interpunct as a decimal point (though its style guides do not specify a multiplication symbol, so presumably '×' should be used when juxtaposition isn't an option, e.g. for scientific notation). Most British schools still teach it this way as well, where the dot product is always a baseline dot. (This convention also used in some other European countries, which use the comma as the decimal separator and the period as the thousands separator. But it's confusing, because 〈x,y〉or even (x,y) is also used to represent the inner product of x and y. It's really a mess.) EebstertheGreat (talk) 04:36, 28 February 2026 (UTC)
- I didn't know it was called the interpunct but that was how I was taught to write decimal points at my UK school in the early sixties and how I still write them by hand.--2A00:23CC:D248:8901:30F4:4052:A4F7:386E 09:36, 28 February 2026 (UTC)
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