Be sure to use the type of vinegar called for in the recipe, or at least one with the same percentage of acidity, so that the finished pickle ends up with a pH below 4.6 and is safe to can in a boiling-water bath. Most of the pickle recipes specify cider vinegar or distilled white vinegar, both of which are generally 5 percent acidity—look on the bottle label to confirm the acidity before you use it—and so can be substituted for one another if necessary. (Cider vinegar, which I think tastes better than distilled white vinegar in most cases, may darken light-colored vegetables a bit.) If the recipe says to use a wine vinegar, again, check the label. Don’t use homemade vinegar for canned pickles, since it’s hard to know exactly how much acid it contains. Also, when a recipe says to boil the vinegar mixture, boil only for the amount of time specified and no longer; acetic acid evaporates more quickly than water does, and excess boiling can throw off the acidity of the brine.