The values in A1:F1 must be sorted for this to work. If the LOOKUP function can't find the lookup_value (i.e. "zzzzzz..."), the function matches the largest value in lookup_vector (A1:F1 in this case) that is less than or equal to lookup_value.

Hence it finds the last item within the array.

A similar formula can be used for finding the last number in a numerical array, i.e.:

=LOOKUP(10E10,A1:F1)

where 10E10 and REPT("z",255) are just very large values.

Hope this helps.

Pete