Block letters and capitalised letters describe different aspects of written text, though they frequently overlap in practical use.

Block letters refer to a style of handwriting or printing where each letter is written individually and clearly, without joining to the next character (unlike cursive or script). The term emphasises legibility through physical separation and distinct letterforms. While block letters can technically be written in either uppercase or lowercase, the term is most commonly associated with uppercase letters, particularly in formal and administrative contexts.

Capitalised letters (or capital letters) refer specifically to the case of individual letters in the alphabet—the uppercase form as opposed to lowercase. Capitalisation is a grammatical and typographic convention applied selectively: at the start of sentences, for proper nouns like names and places, in acronyms, and in titles. A single word may have only its first letter capitalised (e.g., "London") or all letters capitalised (e.g., "NASA").

The key distinction is scope and purpose. Block letters describe how text is physically formed and presented—the writing style. Capitalised letters describe the case of specific characters according to grammatical rules. You can write block letters in lowercase, though this is uncommon; capitalisation, by contrast, applies letter-by-letter according to context.

In practical usage, especially on official forms and documents, "write in block letters" almost universally means write in uppercase, printed characters. This preference exists because uppercase block letters maximise legibility, reduce ambiguity between similar-looking characters, and improve accuracy for both human readers and automated scanning systems like optical character recognition (OCR). However, the definition of block letters itself does not require uppercase—only clear, separate, printed letterforms.