Explanation of :nth-child

gistfile1.txt

div:nth-child(xn+y) {
    /*
    this evaluates like "x*n+y"
    x is cycle size
    n represents a counter, starting at 0
    y is an offset
    */
}

div:nth-child(3) {
    /* third child only */
}

div:nth-child(3n) {
    /*
    every third child
    3*0 (0th child... doesn't exist)
    3*1 (3rd child)
    3*2 (6th child)
    3*3 (9th child)
    */
}

div:nth-child(3n+2) {
    /*
    every third child, offset by 2
    3*0+2 (2nd child)
    3*1+2 (5th child)
    3*2+2 (8th child)
    3*3+2 (11th child)
    */
}

div:nth-child(-n+3) {
    /*
    first three children
    -0+3 (3rd child)
    -1+3 (2nd child)
    -2+3 (1st child)
    -3+3 (0th child... doesn't exist)
    -4+1 (-1st child... doesn't exist)
    ...and so
    */
}