Page "Seatpost" Paragraph 8

Offset or " layback " can range from 0 mm to 45 mm.

A seatpost with offset is necessary when the seat tube angle of the frame is too steep to give the desired saddle setback ( the horizontal distance between a plumb line hung from the nose of the saddle and the bottom bracket spindle ).

Conversely, an " in line " post may be required if the seat tube angle is too slack.

Some saddles, notably Brooks leather saddles, have relatively short rails, allowing less adjustment of setback, and changing the seatpost may be the only way to achieve the correct position.