The shape of each orbital is usually referred to by a letter, rather than by its azimuthal quantum number. The first shape (l =0) is denoted by the letter s (a mnemonic being " s phere"). The next shape is denoted by the letter p and has the form of a dumbbell. The other orbitals have more complicated shapes (see atomic orbital), and are denoted by the letters d, f, g, etc.