Originally Posted by timmhaan

6x6 is 6 repetitions for 6 minutes each. 2X20 is 2 for 20 mins each.

you don't need a heart rate monitor to get benefit out of intervals, however it is useful if you want to target a certain heart rate. for example, this time of year i'm riding 30 minute intervals at tempo, which for me is 152 -158 beats per minute. it would be very difficult to stay in that slim margin without a monitor. for max effort intervals, you can pretty much just pedal as fast as possible, and the HR isn't as important.