Each half-hour of the session gets a letter, and a letter is printed at every price the market touched during that bracket. Stacking the letters horizontally builds the profile shape.
TPO counts measure time at price, which is different from volume-profile, which measures contracts traded at price. The two usually agree but can diverge sharply when a lot of volume trades quickly at one level, and the disagreement is itself informative.
TPO is a product of the pit era, when time brackets were the natural unit and volume at price was not available. With modern data, volume profile is often the more direct measurement, and many traders now use both side by side.
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