To dip a flag that is being carried means to lower it by turning it forward from an upright position to 45° or horizontal. This is done as a sign of respect or deference. At sea, it is done by lowering to half-mast and returning to full mast position.
To dip the flag on a merchant vessel passing a naval vessel involves lowering the stern flag (the country flag) to the half-mast position and back to the truck as the vessels pass abeam of each other. The half-mast position in this case being one flag width from the truck as in the case of half mast. Some jurisdictions have laws that discourage or prohibit the dipping of the national flag; these include the United States (with its non-binding flag code) that allows the dipping of the ensign as a response to a salute from a ship of a foreign nation, India, the Philippines, and South Africa.
- K. V. Singh, Indian tricolor -Flag Foundation of India 2005 - Page 297 "Dipping a flag: Means lowering a flag briefly and then instantly raising it again at sea while sailing past a ship, particularly a warship. It is an old custom to honour, greet and salute an important person or a ship at sea."