What are the different types of cloth used in coated abrasives?

Cloth backings come in various weights, from Stiffest to the most Flexible – (Y, X, J, F)
“Y” Weight – This extremely heavy duty cloth is used for tough applications. It is usually made of Polyester for added strength & tear resistance.
“X” Weight – This heavy cloth, also called drills, is recommended for all ordinary machine sanding operations.
“J” Weight – This flexible, lightweight cloth, also called jeans, is used where more flexibility is needed, such as with contour sanding.
“F” Weight – This highly flexible, lightweight jeans cloth is used when extreme flexibility is desired.