Fullwidth Full Stop

It fits Latin punctuation into an East Asian character grid. The fullwidth period takes one cell, like an East Asian character, while halfwidth forms take less space. In older fixed-width East Asian encodings, that distinction let Latin and East Asian text share a regular grid. Japanese input calls the fullwidth mode zenkaku, so a zenkaku period takes the full cell.

Japanese punctuation was adapted for print. Western-style full stops were advocated in Japanese print during the Meiji period (1868–1912), and modern fiction helped spread punctuation from the mid-1880s. Those milestones describe the adoption of punctuation, not the first appearance of this exact fullwidth glyph.

Where is it a deliberate style choice?

Technical and mixed Japanese-Western text is a documented setting. Japanese layout guidance includes ． in horizontal punctuation for text mixing Japanese and Western writing, including science and technology books. Chinese layout guidance records it in scientific and technical writing, Western-language textbooks, and grammar books, where it helps distinguish a period from the Latin letter o and the numeral 0.

This is a choice of house style: the mark still ends a sentence, but it can signal Western-style or technical typesetting. Horizontal Japanese may use this fullwidth period; vertical Japanese conventionally uses 。.

What does ． mean on its own?

Read it as a punctuation mark first. A bare ． is a fullwidth sentence stop. In chat, the surrounding words and punctuation show whether it was deliberate formatting or an input mismatch.