The Yepp Maxi seattube adapter is necessary to mount the Yepp Maxi rear child bicycle seat (note this adapter is NOT suited for the Yepp Maxi). In the Yepp Maxi - One Set Pack, the adapter is included with the product. A second Yepp Maxi seattube adapter might come in handy to easily switch your Yepp Maxi child bicycle seat between different bikes in one family. After all the Yepp seat is quick and simple to snap into place and detach. This fixture is suitable for bicycles with a frame size of 21-Inch or larger and a seat tube diameter of 28-45-Millimeter/1.1-1.8-Inches. Use the longer bolts provided with the seat for seat post diameters greater than 35-Millimeter/1.4-Inch. Use the Yepp products only on a bicycle that is in good working order and of the type suitable for carrying the additional load. Contact the bicycle manufacturer to obtain this information.