Chain Rings

Mozza1 Posts: 128
edited May 2009 in Workshop
Current chainset is a 9sp Ultegra 53-42 type A.
Want to change the 42t to a 39t which is type B.
Are the two types compatible?
& what is the difference & why did Shimano do two types?
Last thought; could I use a 10sp 39t?