Classics were the first type of extension to be made.  

The base is a circle. like real  human  hair.

The flat lash has a laser-cut base to fit around the curve of the natural lash meaning less adhesive and better retention. 

Flat lashes are 70% lighter than classics. So you can even use 0.20,0.25 thickness with them without having to worry of the health of the your lashes.

Anyway. classic is always The foundation. Flat Lash is such a such a revolution!