Scented liquids go by a number of different names: perfume, cologne, eau de toilette, among others. These aren't just exotic-sounding aliases thought up by advertisers. Each one indicates a ratio of essential oils to alcohol, or concentration, which makes a real difference in how long a scent lingers. The industry hasn't established a standard for concentrations. By definition, however, a perfume is highest, at up to 40 percent essential oils by volume. An eau de parfum is about 15 percent, and an eau de toilette may be as little as 5 percent.
Having a variety of concentrations lets you match the form to the need. You can refresh a perfume for an hour or so by layering it with its eau de parfum version, for example.
When deciding what form to buy or apply, remember that a fragrance's concentration is different from its lightness or heaviness. A perfume composed mostly of light, citrusy notes may need touching up sooner than an eau de parfum with a strong cedar or mossy base.
You can make your own concentrations by starting with essential oils and diluting them as desired. Use a mixture of nine parts alcohol to one part distilled water as the solvent. For a concentration of 20 percent, for example, you would add eight drops of the alcohol-water blend with two drops of the essential oils of your choice.