Many necklaces have extra links of some kind at one end so that the clasp on the other end can attach in different places to adjust the length. The decorative chains that are often used for ends are attractive when they dangle at the back of your neck. Adjustable necklaces are good choice when you're giving a gift and aren't sure which length to buy.
A clasp can add 3/4" or more to length. When you're shopping online read descriptions carefully to find out if clasp lengths are included in the overall length given for each necklace.