If you only chart regular hours (EDT 9:30-16:00) you will have an hour bar that only contains half an hour of data.

1) Should this be the 9:30-10:00 or the 15:30-16:00?
2) What does your charting package do?

My answers:
1) 9:30-10:00 to line up with the hour bars you would get if you charted the whole 24 hours
2) Ensign does 15:30 - 16:00

JustWondering 

If you only chart regular hours (EDT 9:30-16:00) you will have an hour bar that only contains half an hour of data. 1) Should this be the 9:30-10:00 or the 15:30-16:00? 2) What does your charting package do? My answers: 1) 9:30-10:00 to line up with the hour bars you would get if you charted the whole 24 hours 2) Ensign does 15:30 - 16:00

So you are talking about stocks. I would use solution (2). The first bar from 9:30 to 10:30 AM represents your initial balance. You can't display that balance on your chart with solution (1).

Furthermore solution (2) is compatible with the futures market, which has the same bars, but a last bar, which contains 45 min of data, as it starts at 3:30 PM and ends at 4:15 PM. Or would you want to subdivide the RTH session for ES into one 30 min bar, six 60 min bars and one 15 min bar?

For NT you can adjust this via session templates. Solution (2) is easier to display, but you could theoretically force solution (1) by introducing a separate subsession from 9:30 to 10:00, which however would unnecessarily compicate things. So better scrap solution (1).

Hope this answers the question.

Thanks Fat Tails

What I really want is to know what is most widely used since there is a fair bit of self fullfilling prophecy in the intraday markets. Your comment about the futures clinches the matter.