Written from the rule, not from a summary
Most billing explainers on the web are paraphrases of other explainers, which is how an error survives for years: nobody in the chain ever read the source, and the mistake gets more confident with each retelling. Box 24J is the standard example: half the material about it describes the wrong half of the split field.
These guides work the other way round. Where a rule is published, the guide links it in its sources: the NUCC instruction manual for the claim form, the Medicare Claims Processing Manual for timed units. Where a rule genuinely has two readings (and the 8-minute rule does, because Medicare’s version and the AMA rule of eights disagree) both are given, with what each means for your units, instead of picking one and hoping your payer agrees.
They cover form fields and payer rules, which CMS publishes. They contain no CPT code list or descriptors, because that code set is licensed by the AMA and a free guide is not a licence.
Which guide answers your question
A claim came back and you suspect the form. CMS-1500 fields, box by box goes through the form in order and singles out the boxes behind most avoidable denials: the diagnosis list in Box 21, the pointer limit in 24E, and the three NPI fields that billers confuse (24J, 32 and 33). Read it if you prepare claim data for a biller or check what they send back.
A patient asked for paperwork to claim from their insurer. Superbill vs CMS-1500 explains which document goes to the patient and which goes to the payer, and why a receipt is not enough. It is the short one, written for cash-pay and out-of-network practices and for hybrid practices that issue both.
Your unit counts do not match your minutes. The 8-minute rule shows how timed minutes become 15-minute units, why Medicare’s pooled count and the AMA per-service count can differ on the same visit, and which minutes stay out of the total. It is for therapists and billers on PT, OT and speech claims.
When you would rather just get the number
Two of these have a calculator attached. The 8-minute rule calculator takes timed minutes per code and shows Medicare and rule-of-eights results side by side; the superbill generator turns the field explanation into something you can fill in and print. Both are free and run in your browser.
See a note turn into CMS-1500 field data
14 days, no card required. Use made-up patients while you evaluate: real patient information waits until a business associate agreement is in place.