If you're still really new at domaining and you're not too sure about which letters are called 'premium letters', here's how they're categorized:

Premium Letters: A, B, C, D, E, F, G, H, I, L, M, N, O, P, R, S, T
Good Letters: K, L, W, Y
Bad Letters: Q, V, X, Z

I'm pretty sure that's how they're categorized but if anyone feels it's wrong, feel free to let us know.