A device that uses 802.11n High Throughput (HT) mode -- also known as Greenfield mode -- assumes that there are no 802.11a/b/g stations using the same channel. 802.11a/b/g devices cannot communicate with a Greenfield AP. Instead, their transmissions are likely to collide, causing errors and retransmissions for both parties.
Normally, 802.11 devices share channels by sensing when another device is transmitting, using a back-off timer to wait until the channel is free. However, because an 802.11a/b/g device cannot tell that a Greenfield device is transmitting, it will go right ahead and transmit. To avoid this, the 802.11n standard also defines an HT Mixed mode.
A device using HT Mixed mode prepares to transmit in both the old 802.11a/b/g fashion and the new 802.11n fashion. Specifically, HT Mixed mode devices transmit a legacy format preamble, followed by an HT format preamble. An HT Mixed mode device must also send legacy format CTS-to-Self or RTS/CTS (Request to Send/Clear to Send) frames before transmitting. These "protection mechanisms" let nearby 802.11a/b/g devices -- including those not connected to the HT Mixed AP -- sense when the channel is busy.
Of course, these protection mechanisms significantly reduce an 802.11n WLAN's throughput. But they are a price that must be paid to coexist peacefully with 802.11a/b/g neighbors.