Start and Stop bits are used.
A stop bit is the last bit of the communication frame and also the level at which it remain when idle.
A start bit is the first bit of the communication frame and it's always at the opposite level of the stop bit.
For example:
1. If the line is idle, and idle is defined as logic '1' - the first bit of the next frame will always be logic '0'.
2. After that, an X number of payload bits will be sent.
3. At the end the line will sent a stop bit (logic '1') and remain in that level, until the next frame is sent with its start bit.