ICD-10-CM symbols and notes, decoded

ICD-10-CM symbols and notes, decoded: brackets, NEC, NOS, Excludes1, Excludes2 and the placeholder X

Last reviewed: November 2026 (FY2027 guidelines, in effect October 1, 2026 to September 30, 2027)

The conventions in Section I.A of the ICD-10-CM Official Guidelines are the general rules for using the code set, and they're built into the Alphabetic Index and Tabular List as instructional notes. HIPAA requires following the guidelines when assigning diagnosis codes, so these aren't style tips. A missed "code first" or a misread Excludes note is how a correct diagnosis turns into a wrong claim. Here is each one in plain words, with a short example.

Punctuation

  • [ ] Brackets. In the Tabular List, brackets hold synonyms, other wording, or an explanation. In the Alphabetic Index, brackets mark a manifestation code (more on those below).
  • ( ) Parentheses. These hold "nonessential modifiers": extra words that may or may not be in the record. They don't change the code. They appear in both the Index and the Tabular List.
  • : Colon. In the Tabular List, a colon follows a term that isn't complete yet. One or more of the words after the colon must be documented for the code to apply.

NEC and NOS

  • NEC, "not elsewhere classifiable." It means "other specified." The record gives detail, but no code exists for that detail, so it goes to the "other" code.
  • NOS, "not otherwise specified." It means "unspecified." The record doesn't give enough detail to pick a more specific code.

A simple way to keep them apart: NEC is "we know more, but there's no code for it." NOS is "we don't know more."

Includes, Excludes1, Excludes2

  • Includes. Appears right under a three-character category title to further define the category or give examples of what's in it. Separately, some codes list inclusion terms, and the guidelines say those lists aren't necessarily complete.
  • Excludes1: "NOT CODED HERE!" The excluded code should never be used at the same time as the code above the note. One use is when two conditions can't occur together, like a congenital form and an acquired form of the same condition.
  • Excludes2: "Not included here." The excluded condition isn't part of this code, but the patient may have both at the same time. You may report both codes together, when appropriate.

Two examples from the Tabular List (check your own FY2027 book, since notes can change each October):

  • Under J45 Asthma, an Excludes1 note lists wheezing NOS (R06.2). If asthma is coded, the symptom code for wheezing NOS isn't added, since wheezing is part of asthma.
  • Under J01 Acute sinusitis, an Excludes2 note lists chronic sinusitis (J32.0 to J32.8). A patient can have acute sinusitis and chronic sinusitis at the same time, so both may be coded.

The Excludes1 exception. When the two conditions are unrelated to each other, both may be reported even with an Excludes1 note. If it isn't clear whether they're related, the guidelines say to query the provider.

Etiology and manifestation: "use additional code" and "code first"

Some conditions are coded as a pair: the underlying cause (etiology) and how it shows up in a body system (manifestation). The etiology code has a "use additional code" note, and the manifestation code has a "code first" note. The etiology goes first.

Codes with "in diseases classified elsewhere" in the title are manifestation codes. They are never allowed as the first-listed or principal diagnosis.

Example: F02.80, dementia in other diseases classified elsewhere, unspecified severity, without behavioral disturbance, psychotic disturbance, mood disturbance, and anxiety. Its category, F02, carries a "code first the underlying physiological condition" note, such as Alzheimer's disease (G30.-). Code the Alzheimer's first, then the dementia code.

Code also

A "code also" note says two codes may be needed to fully describe a condition. Unlike "code first," it doesn't tell you which one goes first. Sequencing depends on why the patient was seen and the sequencing rules.

"And", "with", "see" and "see also"

  • "And" in a code title means "and" or "or." A code for "A and B" can be used for A alone, B alone, or both.
  • "With" (or "in") in a title means "associated with" or "due to." The classification presumes a causal link between conditions joined by "with" in the Index or Tabular List, unless the record clearly says they aren't related, or another guideline requires the provider to document the link.
  • "See" in the Index sends you to another term. You must go there to find the code.
  • "See also" in the Index means another main term may also help. You don't have to follow it if the main term you're on already gives the code.

Placeholder X and the 7th character

  • Some categories require a 7th character, and every code in that category needs it (or as the Tabular notes say).
  • Some codes use X as a placeholder to allow for future expansion. Where a placeholder exists, the code isn't valid without it.
  • If a code that needs a 7th character is shorter than 6 characters, fill the empty spots with X too.
  • Example: T36.0X1A, poisoning by penicillins, accidental (unintentional), initial encounter. In categories T36 to T50, the X is the placeholder held for future expansion in the 5th spot. The 1 (accidental) is the 6th character and the A is the 7th. The code isn't valid without the X.

"Other" and "unspecified" codes

  • "Other" or "other specified" codes are for when the record has detail but there's no specific code for it.
  • "Unspecified" codes are for when the record doesn't have enough detail for a more specific code.

When an unspecified code would be the only choice and you think the record could say more, that's often a query, not a guess.

Why this matters past the coding desk

These notes show up on real claims. Two codes that an Excludes1 note says not to use together (and that aren't unrelated) shouldn't be on the same claim. A code with "in diseases classified elsewhere" in its title should never be listed first. A code missing its X placeholder isn't a valid code. If you work in billing or denials, reading a claim's codes with these rules in mind can help explain why it came back.

Want it on two pages?

Our ICD-10 Conventions at a Glance sheet puts the Section I.A conventions and the Section I.B general rules on two printable pages, with section tags so you can find the guideline fast: ICD-10 Conventions at a Glance

Free help

Not a coder, and here because of a denial? See our Who will help me for free? page for free help by type of coverage, or the free Denial Letter Decoder.

Sources

This is general educational information, not legal or medical advice, and not a substitute for the Official Guidelines or your organization's policies. Always follow the current code book and guidelines.

AI disclosure: This post was drafted with help from AI tools, then reviewed and fact-checked by our founder, a certified medical coding professional.

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