Pearl strands typically come in the following categories.  

Choker - 16 inches -  A pearl choker is perhaps the most classic and yet versatile of the entire single strand lengths. A simple pearl choker can go with virtually any outfit from casual to fancy eveningwear, and just about any neckline imaginable.

Princess - 18 inches - The princess length necklace is best suited for crew and high necklines. It also complements low plunging necklines.

Matinee - 20 to 24 inches - Longer than the choker, and just a bit shorter than an opera length, the matinee necklace is the right choice for casual or business dressing.  When buying for an older woman, a longer length helps to draw the attention away from the neck and to the pearls.

Opera - 28 to 36 inches - The opera necklace is the queen of all the lengths. When worn as a single strand, it is refined and perfect for high or crew necklines.  When doubled upon itself, it serves as a versatile two-strand choker.