The handkerchief and the pocket square are both important elements in a man's attire. These items are similar, but they have different uses.
A handkerchief is a small piece of fabric that is used for blowing your nose or wiping sweat from your face. Handkerchiefs can be made from a variety of fabrics, but their use suggests that a sturdy, soft, inexpensive fabric is ideal. A man carries his handkerchief in his pants pocket.
The Pocket Square
Like the handkerchief, a pocket square is a small piece of fabric. The use of the pocket square, however, is only for looks. A man wears a pocket square in his suit jacket breast pocket, with a portion sticking out for a fashionable addition to his attire.
Wearing a Pocket Square
There are a variety of ways to coordinate your pocket square with the rest of your suit. The square should provide subtle contrast so that it is not the primary focus of the ensemble. A variety of textures provides interest. If you are wearing a tie made of heavier fabric or a rough tweed jacket, try a silk pocket square. Wool suits look nice with linen squares.
When choosing a color, coordinate your pocket square with a secondary color in your ensemble. Look for a subtle color in your printed tie, or a deeper version of your shirt, or a lighter shade of your jacket. According to Antonio Centeno of online men's retailer A Tailored Suit, the only true rule is that your pocket square should not be an exact match of your tie. If choosing an appropriate color or texture seems daunting, a solid white cotton or linen pocket square is a suitable choice for almost any ensemble.
A handkerchief and a pocket square have a number of similarities. As mentioned, they are both small squares of fabric, and both can be made from a variety of fabrics. If fact, the origin of the pocket square is in the handkerchief.
It is in the uses of the handkerchief and the pocket square that the differences lie. Handkerchiefs are meant to be used for your hygienic needs. Pocket squares should never be used for this purpose and are strictly for show. Ideally, a gentlemen will have both a pocket square for fashion and a handkerchief for function.