R - Commands  
RAY (command)
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It creates a semi-infinite line

 Command line: RAY

Specify starting point: Specify a point

Specify a point for the ray to pass through, or the semi-infinite line angle or else E to finish

The ray is extended to the edge of the display in the direction defined by the starting point and the through point. The prompt for a through point is redisplayed so that it is possible to create multiple rays. Press ENTER to end the command.