Final answer:
In medical coding, such as ICD-10, the letter 'X' is used as a placeholder when a code does not have six characters, ensuring the seventh character extender remains in the correct position.
Step-by-step explanation:
The placeholder used when a code does not have six characters in order to keep the seventh character extender in the seventh position is X. This placeholder is commonly used in medical coding, such as ICD-10 codes, where it's vital that each character is placed in a specific position to maintain the code's integrity and meaning.
If a given code has fewer than six characters and a seventh character is required, the placeholders are filled with the letter 'X' so the seventh character extender can be accurately positioned.