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Assuming you speak of UART, then the receiver must be told in advance the number of data bits, the number of parity bits and the number of stop bits. The start bit is a logic zero (low) and the stop bit(s) are logic ones (high), which is also the idle state.
The data bit stream itself isn't asynchronous, only the time between bytes is. Asynchronous in this context rather means "no separate clock signal provided".
The receiver needs to know if partiy is used but also what baudrate that is to be expected. It will clock from the edge of the start bit to where the stop bit(s) are expected. If an incorrect signal level is encountered where the stop bit(s) are supposed to be, then it is a so-called _framing error_ and the UART controller will error flag.
There's a de facto standard nomenclature used to describe UART communication format:
`baudrate/databits-partiy-stopbits`
For example one would say `9600/8-N-1` and mean baudrate 9600bps, 8 data bits, no parity, 1 stop bit. In case of parity, the letter would be `E` or `O` for even or odd, although parity is mostly regarded as an obsolete form of error detection and not often used nowadays.

Assuming you speak of UART, then the receiver must be told in advance the number of data bits, the number of parity bits and the number of stop bits. The start bit is a logic zero (low) and the stop bit(s) are logic ones (high), which is also the idle state.
The data bit stream itself isn't asynchronous, only the time between bytes is. Asynchronous in this context rather means "no separate clock signal provided".
The receiver needs to know not only if parity is used but also what baudrate is to be expected. It will clock from the edge of the start bit to where the stop bit(s) are expected. If an incorrect signal level is encountered where the stop bit(s) are supposed to be, then it is a so-called _framing error_ and the UART controller will error flag.
There's a de facto standard nomenclature used to describe UART communication format:
`baudrate/databits-partiy-stopbits`
For example one would say `9600/8-N-1` and mean baudrate 9600bps, 8 data bits, no parity, 1 stop bit. In case of parity, the letter would be `E` or `O` for even or odd, although parity is mostly regarded as an obsolete form of error detection and not often used nowadays.