At what line of longitude is the International dateline?
http://en.wikipedia.org/wiki/International_date_line
180 degrees
To find the line of longitude at which the International Date Line lies, you can refer to reliable sources such as the Wikipedia article on the International Date Line. According to the article, the International Date Line is located at approximately 180 degrees longitude.
The International Date Line deviates from the 180 degrees longitude to accommodate various political and geographical boundaries. Additionally, there are a few small deviations along this line to ensure that certain regions remain on the same day.