This had been discussed and decided on IRC.
Rationale:
Therefore the space is recommended in the SI/ISO 31-0 standard, and the
International Bureau of Weights and Measures states that "for numbers with
many digits the digits may be divided into groups of three by a thin space, in
order to facilitate reading. Neither dots nor commas are inserted in the
spaces between groups of three".
(http://en.wikipedia.org/wiki/Decimal_separator#Digit_grouping)
(integer :tag "Number of characters")
(float :tag "Fraction of window")))
-(defcustom notmuch-hello-thousands-separator ","
+(defcustom notmuch-hello-thousands-separator " "
"The string used as a thousands separator.
-Typically \",\" in the US and UK and \".\" in Europe."
+Typically \",\" in the US and UK and \".\" or \" \" in Europe.
+The latter is recommended in the SI/ISO 31-0 standard and by the
+International Bureau of Weights and Measures."
:group 'notmuch
:type 'string)