the offset refers to the actual seatpost. A zero offset post has the seat clamp directly above the post part. Like this one:Robbo_ said:How do you work out zero offset, ( I know that probably sounds stupid) from what point?
).
http://www.eastonbike.com/PRODUCTS/POSTS/post_ec90_zero.html
Most road post have some setback: http://www.eastonbike.com/PRODUCTS/POSTS/post_ea70_'06.html
some tri seatposts even have the clamps forward of center. (points forward)
http://www.profile-design.com/2006_product_pages/seat_post/fast_forward.html
most cycling race competitors (apart from the 'unusually dimensioned') are not supposed to have the tip of the seat less than 5cm behind the middle of the crank axle, but triathletes are exempt from this rule.