Newer "Superglide" chainrings from Shimano use similar features to the rear Hyperglide system to improve front shifting. The outer rings have special shaped teeth here and there, called "gates" which make it easier for the chain to climb up. There are also steel pins rivetted into the sides of the larger rings to help the chain climb.
When this system is working as it shoud, the chain makes a very smooth transition from small to large chainring with no slippage. For this system to work as smoothly as it is designed to, the locations of the teeth on the smaller ring should be specifically placed with regard to the positions of the teeth of the larger ring.
This requires that the chainrings be used in matched sets. Shimano has two "road" sets, designated "A" and "B".
- Set "A" uses a 42 tooth small ring, and a corresponding "A" type 53.
- Set "B" uses a 39 tooth small ring, and a choice of "B" type 52 or "B" type 53
Only Shimano is this fussy about chainwheel matching, and this is by no means mandatory. In particular, if your bike currently has an "A" set 53/42 and you decide you'd rather have a 39, this doesn't mean you need to replace the 53 as well. It will still shift, and shift well, just, perhaps, not quite as well as the designated set.