What is the right word to describe the end of a (phone) number?

I know suffix is used for words (letters), but I'm unsure if it is the right word for numbers.

Example with a number:

34 is a ____ of 1234

... or with a phone number:

1234 is a ____ of +972500001234

I'm not sure the fact that its a phone number matters but maybe it does.

  • 2
    Example please. I have no idea what you consider as the end of a phone number or indeed which end you are thinking of. Do you mean the country code? If not you may be thinking that the structure of phone numbers is identical in all countries. It isn't.
    – David
    Nov 25 '18 at 13:48
  • Other than an extension number, why would you ever put a "suffix" on a phone number. What do you mean? Give examples!
    – Hot Licks
    Nov 25 '18 at 13:51
  • 3
    "The last two digits" and "the last four digits".
    – Hot Licks
    Nov 25 '18 at 14:31
  • 1
    Bear in mind that suffix is not just "the end of a word"!
    – oerkelens
    Nov 25 '18 at 15:35
  • 1
    Are you asking about an arbitrary number of final digits, or a specific (case-by-case) number?
    – Spencer
    Nov 25 '18 at 19:18

In a general context of string processing in programming, I would call the substring that appears in the end a postfix. Collins has postfix as a synonym for suffix, while defining suffix as

one or more numbers or letters added to the end of a code number to indicate, for example, what area something belongs to.

To compare, a substring that appears in the beginning would be a prefix, and a substring in the middle would be an infix.


That it is a phone number does make a difference.  Phone numbers are encodings, and each section has a meaning and a purpose. 

1-800-555-1212 x1234

I would label the sections of this phone number in this manner:

1-800-555-1212 x123
| |   |   |     |
| |   |   |     extension (if any)
| |   |   |     
| |   |   line number
| |   |   
| |   exchange 
| |   
| area code

However, the above is an American phone number format, and it assumes dialing on an outside line. 


I might guess that the +9 is a prefix for reaching an outside line from within a local PBX, but if so I can't parse the rest of it with any confidence.  The number of digits is just odd. 

Or, that might be the +972 country calling code prefix for Israel.  If so, I'm tempted to parse it this way:

+972 50 000-1234
 |   |  |   |   
 |   |  |   |   
 |   |  |   |     
 |   |  |   line number
 |   |  |   
 |   |  exchange 
 |   |  
 |   area code
 country code

I've answered, but I don't think this is an on-topic question for this site. 

  • 2
    Trouble is you can’t expect all phone numbers to be structured the same way. For example, +46 40 341 937 and +46 73 739 3725 are both (fictitious) Swedish phone numbers, but only one of them contains an area code. Nov 25 '18 at 19:57
  • Note that the grouping into "exchanges" is a historic limitation of the North American Numbering Plan. Most other countries' numbering plans do not have the notion of "exchanges". The ITU-T recommendation E.164 for an international numbering scheme really just distinguishes between a country code and a subscriber number. It is then up to the regulatory body of each country to structure the subscriber number. It's common to have area codes (referring to "numbering plan areas", not necessarily geographical areas, @JanusBahsJacquet), but even that is not universally the case.
    – n.st
    Nov 25 '18 at 20:34
  • 1
    In fact, phone numbers are so diverse and complicated to parse that there is special software for it (and it contains tens of thousands lines of code and relies on a massive database of special cases).
    – n.st
    Nov 25 '18 at 20:39

As mentioned in a comment, if a certain institution has several numbers beginning with +97250000, distributed around several departments, the last four numbers are telephone extension numbers. In your example it would be extension "1234".

  • The number assigned within an organization to an individual telephone that extends the external telephone number. (NCI Thesaurus)
  • Thanks for the reply. However, this is not what I meant. I do not know if the "suffix" I refer to is used to separate extensions in an organization or not. It is an arbitrary suffix of the phone number used for my own purpose. It is unlikely it is even an organization behind it. Just a regular, private-use, phone number Nov 25 '18 at 15:38
  • 1
    @ReutSharabani What makes you think there should be a word for that? Just like there’s no word for ‘the last X letters of any given word’, there’s no word for ‘the last X digits of any given number’. Nov 25 '18 at 15:55
  • X is meaningless. Any suffix of a number. In my case a telephone number. There maynot ne a word for it, or as mentioned in a different answer suffix may be acceptable. Thank you for your input... Nov 25 '18 at 21:12

I think "suffix", rather than "postfix", is the word you want. In discussions of algorithms, the latter portion of any string is a suffix, irrespective of whether the string consists of digits, letters, or any other combination of symbols. See, for instance, the Wikipedia article on suffix trees. It sounds like it isn't really important that the strings you are working with are phone numbers, so I think you should go with the general term.

"Postfix", on the other hand, is an adjective describing a mathematical operator that goes after its operands.

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