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All the seven segment displays I’ve seen so far have their decimal point to the right of the main digit. Why is this so? A decimal point to the left will be able to represent more possible combinations of numbers.

A single digit display with a decimal point on the left can represent nine more possible numbers (\$ .1,\ \ldots ,\ .9 \$) than the common display with decimal point at the right.

Edit: Why also do the digits lean towards right?

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    \$\begingroup\$ So you can put a decimal point to the right of your number wherever it is. \$\endgroup\$
    – Voltage Spike ♦
    Sep 4, 2019 at 15:20
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    \$\begingroup\$ FYI I've seen 7 segments with decimal points on both the right and the left, but not just the left. \$\endgroup\$
    – Bort
    Sep 4, 2019 at 15:26
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    \$\begingroup\$ Perhaps because there's room for the DP at the bottom right and none at the bottom left. \$\endgroup\$
    – mike65535
    Sep 4, 2019 at 15:29
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    \$\begingroup\$ many displays lean the digits right, to give an italic effect. That leaves more room bottom right than left. \$\endgroup\$
    – Neil_UK
    Sep 4, 2019 at 15:39
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    \$\begingroup\$ @Atom - Maybe it was to leave room for the decimal point :) \$\endgroup\$
    – Bort
    Sep 4, 2019 at 15:45

4 Answers 4

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I think mike65535 is probably correct, more-or-less.

Of course the argument that there's room on the right of the digit and not the left only makes sense if the digit is slanted to the right. There's some discussion of that over here. What's missing from that discussion is that a slight rightward slant gives characters that are slightly more similar to handwriting.

The idea that one can get another power-of-ten out of a given display by putting the point on the left makes sense, but I don't think it would be a good design decision unless you were already constrained to just one or two digits.

A "bare" decimal (without the leading zero) isn't as easy to read, is more likely to be mis-read, and will always be at the low end of the displayable range. Even if we did have the decimal point on the left, it would still almost always be the rigth decision to pay the cost & space for another display digit.

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    \$\begingroup\$ >>A "bare" decimal (without the leading zero). YES! We engineers were all taught to never write a value without the leading zero for the very reason you cite. \$\endgroup\$
    – mike65535
    Sep 4, 2019 at 15:48
  • \$\begingroup\$ and non-engineering users might not know that .25 is 0.25 and all sorts of bad things happen. Also, I saw applications where a seven segment display's DP is blinking to indicate operation or progress... \$\endgroup\$
    – dlatikay
    Sep 6, 2019 at 6:07
  • \$\begingroup\$ Also, if the whole display has to cover that large of a range, it would probably be easier, more efficient and more maintainable to just introduce scientific notation, thus making the leftmost dot a moot point (pun not intended). \$\endgroup\$
    – hoffmale
    Sep 7, 2019 at 6:15
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enter image description here

Figure 1. Vertical display. Source.

enter image description here

Figure 2. Slanted display. Source.

Reasons for slant:

  • '7' in particular, '4' to a lesser extent and perhaps '2', '3', '5', '6' and '9' all appear more natural and resemble print typeface numerals more closely. The others look like italics which we are used to reading and the eye seems to accommodate this quite comfortably.

Reasons for decimal on the right:

  • There's more room there due to the slant. (See above.)
  • Decimal numbers < 1 should always be written with a leading zero. The leading zero is a visual clue that a decimal is being displayed even if the observer doesn't notice it.

Personally, I find 'square' displays a little disconcerting.

Edit by Michael Karas

The idea to rotate one seven segment display 180° to create a colon for a clock, as posed in the comments, is just not a usable solution in my estimation. I have edited this here to be able to show why.

enter image description here

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    \$\begingroup\$ Have you ever read Blindsight by Peter Watts? \$\endgroup\$
    – DKNguyen
    Sep 4, 2019 at 18:58
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    \$\begingroup\$ I'm referring your finding orthogonal things disconcerting. \$\endgroup\$
    – DKNguyen
    Sep 4, 2019 at 21:28
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    \$\begingroup\$ Also if you rotate some segments 180° you can make a colon which works nicely for a clock display. \$\endgroup\$
    – ratchet freak
    Sep 5, 2019 at 9:13
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    \$\begingroup\$ @DKNguyen the Wikipedia article doesn't mention the vampires' orthogonal vulnerability at all, unfortunately. TVTropes mentions it though: "Vampires, because of their unique spatial reasoning, get seizures when they see too many right angles. It gave rise to the myth that they're vulnerable to crosses, but the truth is they short out if they see something as simple as a window pane or a building with a square footprint. They largely went extinct after the invention of architecture in early human history." \$\endgroup\$
    – Ross Presser
    Sep 5, 2019 at 17:33
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    \$\begingroup\$ @ratchetfreak - The idea to rotate a 7-seg display 180° to create a "colon" for a clock looks pretty unusual in my perception. I have added a picture to Transistor's excellent answer to illustrate this point. \$\endgroup\$
    – Michael Karas
    Sep 6, 2019 at 10:24
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Punctuation ,as a typesetting and -writing tradition ,is always placed immediately after a character ,not before .We are just used to see it that way .Note that there are also alphanumeric displays ,not just numeric ones .

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  • \$\begingroup\$ And spaces are placed after the punctuation not before, too. \$\endgroup\$
    – Nappy
    Sep 6, 2019 at 8:59
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    \$\begingroup\$ I assume it was written that way to emphasise the point. \$\endgroup\$
    – user131342
    Sep 6, 2019 at 9:30
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I have an hob where each hot plate uses a single digit. The dot is used to indicate .5 or "point something". I think that range is somewhat more useful than one half in [0..1] and the other in [1..9].

To elaborate more on this, because not everyone seems to get my point:

If you see that use case valid (as I think you should because its an actual product) the assumption that the dot on the left would increase the number of possible values (as done in the question) is not correct. With the dot on the left you get:

(.0), 0 ,.1, .2, .3, .4, .5, .6, .7, .8, .9, 1, 2, 3, 4, 5, 6, 7, 8, 9

With the dot on the right:

0, 0., 1, 1., 2, 2., 3, 3., 4, 4., 5, 5., 6, 6., 7, 7., 8, 8., 9, 9.

So you have the same amount of different states / numbers, but with the latter range the numbers are evenly spaced, while the first range is split between two resolutions: .1 and 1.

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  • \$\begingroup\$ The numbers go in order "0", "0.", "1", "1.", "2", "2." etc \$\endgroup\$
    – zmarties
    Sep 5, 2019 at 22:58
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    \$\begingroup\$ I'm not completely sure what "one half in [0..1] and the other in [1..9]." means. But seems more like a comment than an answer. Maybe I just don't understand. \$\endgroup\$
    – Bort
    Sep 5, 2019 at 23:17
  • \$\begingroup\$ My hob at home does this. 1.5 is represented by 1. \$\endgroup\$
    – Harry Beadle
    Sep 6, 2019 at 10:50
  • \$\begingroup\$ @Bort I think he means that of the 20 numbers that can be displayed with one of 10 digits and a dot, putting the dot to the left gives you 10 numbers between 0 and 1 (with the dot showing) and 10 numbers between 1 and 9 (without the dot). That isn't strictly true as "0" and ".0" are the same number, but I think that's the gist. By contrast, putting the dot to the right to mean ".5" gives 20 numbers equally spaced between 0 and 9.5. \$\endgroup\$
    – Philip C
    Sep 6, 2019 at 15:08

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