Serial Bit Order

A serial bit stream, sent along a communications line, is typically produced by a hardware or software Universal Asynchronous Receiver/Transmitter (UART). If one looks at the physical layer waveform for either an asynchronous or a synchronous serial bit stream using an oscilloscope, then one may notice that things are not quite as one might at first expect. There are two things that could be unexpected:

A line driver might invert the signal prior to transmission (line receivers can do an inversion on reception), the signal that appears on the wires can be upside down (as in EIA-232, and EIA-432). That is a "1" or "idle" period is represented by a low voltage and a "0" or "start bit" is represented as a high voltage.
Ever since the start of asynchronous communications, UARTs have sent bauds (representing bits) down the wire with the least significant data bit first and the most significant bit last. These sequences of bauds are sually sent and received using a shift register, so all that this really implies is a change in the order of the connections to the transmit and receive shift registers.

So, what when you look at an EIA-232 signal on an oscilloscope, you might see:

Differential transmission, such as EIA-485, uses two lines to carry each baud. This carries both the inverted and non-inverted waveforms. LSB-first byte ordering is still commonly used.