Router G series follow this rule:
The IEEE's 802.11g standard is designed as a higher-bandwidth - 54M bit/sec - successor to the popular 802.11b, or Wi-Fi standard, which tops out at 11M bit/sec. An 802.11g access point will support 802.11b and 802.11g clients. Similarly, a laptop with an 802.11g card will be able to access existing 802.11b access points as well as new 802.11g access points.
Router N series follow this rule:
802.11n can operate on either the 5 GHz frequency at full speed or on the 2.4 GHz frequency in "mixed mode" which will support systems only capable of using 802.11b or 802.11g, but it will slow the entire network down to the maximum speed of the earliest standard connected.