Rule 20 is how a boat sailing close-hauled towards an obstruction (a shore, a moored boat, a starboard-tack boat she cannot cross) gets out from under a boat to windward on the same tack.
20.1 Hailing
Hail "Room to tack". You may only hail if you are close-hauled or higher, and you are approaching an obstruction that will soon force you to make a substantial course change to avoid it safely. You may not hail at a mark if a boat fetching it would have to change course as a result.
20.2 Responding
The hailing boat must give the hailed boat time to respond. The hailed boat must respond, even if she thinks the hail was improper: either tack as soon as possible, or reply "You tack" straight away and then give the hailer room to tack and avoid her. Once the hailed boat responds, the hailer must tack as soon as possible. From the hail until the hailer has tacked and avoided the hailed boat, rule 18.2 does not apply between them.
If you think the hail broke 20.1, respond and then protest.
20.3 Passing On a Hail to an Additional Boat
A hailed boat that intends to tack may in turn hail a boat further to windward, even if she would not otherwise be entitled to hail. The same response rules apply down the line.
20.4 Additional Requirements for Hails
In conditions where a hail may not be heard (wind, distance, a sailor who cannot hear) the boat must also signal clearly, typically by pointing. The notice of race may require another method of communication and boats must then use it.
Common misreadings
"The hail was wrong so I ignored it"
A hailed boat must respond even to an improper hail. Her remedy is a protest, not a collision.
Changed in 2025
2025 specifies the words of the hail, "Room to tack", and lets the notice of race prescribe an alternative communication.