Gemeral ROAD bikes' 

Front derailleurs are designed with an angle intercepting chains by reference of standard chainring ratio 50/39T.

However, when upgrading to other ratios like 50/34T (Compact Cranks ) or 56/44T (TIME race), 

This angle becomes uncomfortable when trying to shift between chainring. 

RIDEA as a professional chainring producer,  has found that an additional angle of 4°will much help solve this problem.

Price is only for one,not a pair, you should confirm you really need what and then go buy