The Kitchin Third, as its name implies, is one third the length of the Kitchin Cycle (aka Business Cycle). The ideal Kitchin cycle is 42 months or 1277.5 days in length, the ideal Kitchin Third is 14 months or 425.83 days.
A Kitchin cycle is made up of nine Wall Cycles, therefore each Kitchin Third is made up of three Wall Cycles. PQ Wall had a general rule of third last and weakest. This goes for the final Kitchin Third in a Kitchin Cycle, but also goes for Wall Cycle #3, #6, and #9, or the final Wall Cycle in each Kitchin Third. The Kitchin Cycle often unfolds in the three Kitchin Third sections, but the Kitchin Third is not typically as distinct as the other cycles.