Hong Kong's government has unveiled a controversial plan which would allow Chinese mainland law to apply in the territory for the first time.
It's part of attempts to streamline operations at the new Guangzhou-Shenzhen-Hong Kong Express Rail Link, which will open next year, costing just under $11bn (£8.45bn).
Under the new plan, passengers will be able to undertake border clearance procedures for both Hong Kong and China successively in one building in West Kowloon, Hong Kong.
And it will be mainland law which will be in force in parts of the terminal, even though it's on Hong Kong soil.
The government says that will be more convenient for passengers, but opponents say it could violate Hong Kong law.
How do people normally move from Hong Kong to the mainland?
For those travelling overland, the process is similar to most border crossings between countries.
At the busiest land crossings, passengers must first clear immigration and customs in one physical jurisdiction, then walk or be driven a fair distance to clear immigration in the other jurisdiction.
Hong Kong immigration procedures are handled on undisputed Hong Kong territory, and Chinese immigration procedures are handled in Shenzhen, which is the mainland.
