Yeah, it is a bit difficult to understand. I've searched the web for explanations and have even discovered inconsistent explanations. Here is how I understand it.

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The position of the returned elements have to equal an+b for every n >= 0. This is because the nth-child selector is supposed to return elements that have an+b-1 elements before it. Therefore, to have an+b-1 elements before it, the element would have to be at position an+b. So, position = an+b position-b = an (position-b)/a = n (position-b)/a >= n (only if (position-b)/a is an integer - no fractions) So, for your examples: (n-2) would select all elements because (n-2) => (position+2) which is >= 0 for all positions (3n-2) would select elements 1,4,7,10 because (3n-2) => (position+2)/3 Another way to look at this is to say, create groups of three and grab the element at -2 (or 1 as 3-2 = 1) This site might be useful: http://gallery.theopalgroup.com/selectoracle/ Trevan Andrew Dupont wrote: > I'm frustrated at the ambiguity of the CSS3 spec on this issue. See > for yourself at (http://www.w3.org/TR/2001/CR-css3-selectors-20011113/ > #nth-child-pseudo). The spec says that a and b "must be zero, negative > integers or positive integers," but does not give any examples in > which b is negative -- neither in the spec nor in the test suite. > > Firstly: I can only assume that if b is negative the syntax is, for > instance, (3n-2) instead of (3n+-2), as the latter would just be > silly. > > Secondly: If I assume the same sort of logic for negative values of b > as for positive values, then out of a group of 10 elements: > > (n-2) selects elements 1-8 (just like n+2 selects 2-10 and -n+2 > selects 1-2) > (3n-2) selects elements 1, 4, and 7 (the -2 value of b determines the > modulus and also excludes the last two items in the collection) > (n-9) selects element 1 > > Here's a pastebin of the rewritten logic. (http://pastie.caboo.se/ > 47481) Note that the "total" argument specifies the length of the > collection (so that it returns only indices that actually fall within > the result set). > > Thoughts on either of these issues? > > Cheers, > Andrew > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Prototype: Core" group. To post to this group, send email to prototype-core@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/prototype-core?hl=en -~----------~----~----~----~------~----~------~--~---