This is usually the freshly squeezed juice of the Persian or Tahitian lime (Citrus latifolia), though the juice of the Key lime (Citrus aurantifolia) may also be used. The Persian lime, seedless and with a thin rind, is the standard lime used in American bars for fresh juice and for garnish. The Key lime is smaller and has a tarter, more bitter flavor than the Persian lime.
Some bartenders use Rose's lime cordial in place of fresh lime juice; however, Rose's and fresh lime juice do not make good substitutes for each other. A good recipe should specify which ingredient is to be used. If a recipe calls for "fresh lime juice," it is advisable not to use Rose's.