The power of attorney (often called "P O A" for short) may be oral -- such as asking someone else to sign your name on a check because your arm is broken -- or may be in writing. Many institutions -- such as hospitals, banks, and the I.R.S -- require a power of attorney to be in writing before they will honor it (and they usually want to keep an original for their records). The "equal dignity rule" is a principle of law that requires a document authorizing someone representing someone else to have been appointed with the same formality as required for the act the representative is going to perform, and it applies to powers of attorney: That means, for example, that if you give someone your power of attorney to sign the papers to sell your house, and the law requires that signature on the deed to be notarized, then your power of attorney authorizing that attorney in fact to sign the deed must be notarized, too.
A power of attorney may be "special" or "limited" to one specified act or type of act, or it may be "general," and whatever it defines as its scope is what a court will enforce as being its scope. (It may also be limited as to time.) Under the common law, a power of attorney becomes ineffective if its grantor dies or becomes "incapacitated," meaning unable to grant such a power, because of physical injury or mental illness, for example. Under the laws of the U.S. states, however, a grantor may specify that a power of attorney will continue to be effective even if the grantor becomes incapacitated (but it will still end when the grantor dies) and is called a "durable power of attorney." In some jurisdictions such a durable power of attorney can also funcation as a "living will," which can be used to appoint someone to make health-care decisions for the grantor, up to and including "pulling the plug" on machines keeping them clinically alive. New York State has enacted a "Health Care Proxy" law that requires a separate document be prepared appointing one as your health care agent. Unless the power of attorney has been made "irrevocable" (by its own terms or by some legal principle), the grantor may "revoke" (= cancel or "take back") the power of attorney by telling the attorney in fact it is revoked, however if the principal does not inform third parties and it is reasonable that the third parties could rely upon the power of attorney being in force the principal may still be bound by the acts of the agent, though the agent may also be liable for such unauthorized acts.
Many standardized forms are available (usually for free) for various kinds of powers of attorney, and many organizations provide them for their clients, customers, patients, employees, or members. In some states statutory power of attorney forms are available as some individuals have used powers of attorney to unscrupulously waste that assets of vulnerable individuals such as the elderly (see elder abuse).