资料备查：Proof of citizenship
Proof of citizenship
If you are from the United States, the following situations may apply to you:
If you are a citizen or permanent resident of the U.S., you do not need a passport or visa to be admitted to Canada. However, you should carry evidence of citizenship, such as a birth certificate, certificate of citizenship, or certificate of naturalization. If these documents do not contain a photograph, you may also be asked for a driver’s license or other identification. Permanent residents of the U.S. who are not citizens should carry their Resident Alien Car (I-1512 or I-551).
There are more reasons behind the 6-month rule. Please check the last requirement in the article below. I found it at http://www.murthy.com/travel.html
There are a number of criteria that must be reviewed to determine if a person is eligible to apply for United States (U.S.) citizenship. As a starting point, the applicant must be a Legal Permanent Resident (LPR), and at least eighteen years old. There are limited exceptions to this rule, including honorable service in the U.S. military during a time of war or declared hostility. However, the basic rule is LPR and eighteen years of age.
CONTINUOUS RESIDENCE REQUIREMENT
In order to be eligible for naturalization, after LPR status has been obtained, the individual must be able to establish “continuous residence” in the U.S. for a period of five years before filing the application. This period is reduced to three years for individuals who are married to U.S. citizens or who obtained LPR status based upon marriage but were battered or abused by their spouses. With the exception of abuse in order to be eligible for the three-year period based upon marriage, the applicant must be “living in marital union” with the U.S. citizen spouse. The five- or three-year period is referred to as the “statutory period.”
PHYSICAL PRESENCE
Continuous residence does not mean that the applicant must physically remain in the U.S. for the entire statutory period. There are essentially three requirements.
The first of these requirements is that the applicant must be physically present in the U.S. for at least half the statutory five- or three-year period in order to be eligible for naturalization. To determine eligibility, the applicant needs to review the appropriate five- or three-year period immediately preceding his or her application and count the number of days s/he was physically in the U.S. The number of days spent in the U.S. must equal at least one half of the total days in the five- or three-year period in order to be eligible to naturalize.
The next requirement is that no individual trip outside of the U.S. can be of one year’s duration or greater. Trips of this duration are considered to automatically break the physical presence requirement. A reentry permit does not overcome this problem. In the event that the applicant was outside of the U.S. for one year or more, s/he would have to wait at least four years and one day from the time of return to the U.S. to apply for naturalization.
Third, trips that are over 6 months in duration are presumed to break the continuity of residence. This legal presumption can be rebutted with evidence that the applicant did not abandon his or her residence. The inquiry into whether residence was abandoned will include whether U.S. tax returns were filed, whether the individual maintained a permanent home in the U.S., whether the individual worked abroad, the reason for the absence, and family ties to the U.S. This is in no way an exhaustive list and each case must be argued on its own facts. The safest route is to limit trips outside the U.S. to less than six months, if at all possible. This precaution will eliminate the issue entirely.
