ICD-10 chapters and code letters

I've been thinking about ICD-10 codes; they come up a lot in my work.

The ICD-10-CM standard is divided into 21 chapters, which generally correspond to the first letter of a code. However, a chapter may contain blocks beginning with more than one letter, and codes starting with a single letter, namely D, can span two chapters.

Here's a diagram I made to visualize the relationship between chapters and initial letters of codes.

Notice there's no letter U on the diagram. That's because U is reserved for special/provisional codes.

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