Canada–United States border

The Canada-United States border is the longest undefended border in the world. As boundaries go much of the border is artificial, as usually borders are defined by bodies of water (rivers and lakes) or other geographical features such as mountain ranges. Much of the border is a straight line especially along the 49th parallel. This artificial aspect to the border has made it so that many cross-border socioeconomic connections are stronger between adjoining areas across the border than with other areas of their own country, such as the relationships between New England and Atlantic Canada and Quebec, Michigan, Minnesota and New York with Ontario, the Dakotas and Minnesota with Manitoba, Montana with Alberta and Saskatchewan, Idaho, Washington, and Alaska with British Columbia.

This historical relationship has lead to many hockey leagues being international in nature with American teams playing in many Canadian provincial leagues and lead to a few changes in Canadian championships such as the original Stanley Cup format with teams from Portland, Oregon and Seattle, Washington becoming home to professional teams in the 1910's leading the Stanley Cup Trustees to change who was allowed to challenge for the Cup.

Many junior, senior and minor leagues have have teams play from both sides of the border. American teams have won the Memorial Cup and Allan Cup.

For much of 2020 and 2021 the border was closed as a result of the COVID-19 pandemic leading many leagues to change from traditional scheduling formats to get a season in and in some cases causing outlier teams to have to sit out the season as the single American teams playing in the Kootenay International Junior Hockey League, Ontario Junior Hockey League, Superior International Junior Hockey League, and Greater Ontario Junior Hockey League.