Hi, Barb,
The problem is probably that the uppercase 'Z' sorts above the character(s) that have an accent symbol on them as (eg. as many French characters do). If you're using these characters when entering a customer name, you'll want to use them as the first character of the Customer Name field in the Sort by criteria of the Print Aged Trial Balance report window also.
Or, you could Change the Customer Name field on the window to something else that might be relevant to you, like Territory, or even remove it from the list by changing it to the 'blank' field name setting. This will effectively move it to the bottom of the field criterial list.
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Regards,
Softrak Tech Support