USB communication takes the form of packets. Originally, all packets are sent from the host, using the root hub and possibly more hubs, to components. Some of those packets direct a device to send some packets in reply.
After the sync field discussed above, all packets are made of 8-bit bytes, transmitted least-significant bit first. First byte is a packet identifier (PID) byte. The PID is in fact 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. It redundancy helps detect errors. (Note that a PID byte contains at most 4 consecutive 1 bits, and consequently will never require bit-stuffing, even when combined with the last 1 bit in the sync byte. though, trailing 1 bits in the PID may need bit-stuffing within the first few bits of the payload.)