Picture below shows how the idealized signal looks on an oscilloscope.
MARK is a negative voltage, and SPACE is positive.
Voltage levels defined in the standard
Space = 3 to 15 , Mark = -3 to -15 Volts
"Off" = -3 to -25 , "On" = 3 to 25 Volts
The following is the truth table for RS-232: Signal>3V=0 Signal>-3V=1
The output signal level usually swings between +12 V and -12 V.
The dead area between +3 V and -3 V is designed to absorb line noise.