The main difference between classic and flat lashes is their shape. Traditional classic lash bases are circular, while the base of a flat lash tends to be more oval in appearance. This means that even if we put both types on with equal weight, they'll look different: flats will give you an eyeliner effect (think wispy), whereas classics create quite a natural-looking subtlety when applied correctly.
Another difference between flat and classic lashes is the weight. Flat lashes are lighter than conventional ones. Thus, 0.15mm flat lashes will feel like a conventional lash extension and offer the width of a thicker and heavier set. The weightless quality makes them ideal for clients who have thin or short natural lashes because they won't cause damage to their existing eyelashes while still adding length and volume.