Distress signals include:
a) "SOS" (...---...) signal made by any audible or visual means;
b) International Code Flags: "NC" (a blue/white checkerboard flag above a blue-white-red-white- blue horizontally striped flag);
c) Hoisting any square flag with a ball (or anything resembling a ball) above or below it;
d) Flames made visible (as burning oil barrel);
e) A rocket parachute flare or hand held flare showing a red light;*
f) Rockets or shells, throwing red stars, fired one at a time over a short interval;
g) Continuous sounding of any fog-signal device;
h) Slowly and repeatedly raising and lowering arms outstretched to each side;
i) Signals transmitted by emergency position-indicating radiobeacons (EPIRBs);
j) A signal sent by radiotelephone consisting of the spoken word "MAYDAY";
k) Radiotelephone alarm signal: generally sent over 2182 kHz and consisting of an alternating audio signal sounding something like a siren (BEEEEEE-DOOOOOO, BEEEEEE-DOOOOOO, etc.);
l) A piece of orange-colored canvas with either a black square and circle or other appropriate symbol (for identification from the air);
m) A dye marker of any color.
n) Orange smoke emitted from distress flare.
o) A gun or other explosive signal fired at intervals of about 1 minute.
p) The radiotelegraph alarm signal.
q) In Inland locations, a strobe light.
* As noted above in the Rule 36 section, a red light or flare indicates distress versus a white light or flare which is used for illumination. Also, note that the upside down US flag is no longer a valid distress signal.