Katakana is a fragment of a kanji

"Kata" (片) means partial

Katakana appeared around the same time as hiragana, in the 9th century, but different people made it for a different reason. This one came from Buddhist monks.

Monks had to read Chinese scriptures in Japanese word order, which meant squeezing particles and verb endings into the narrow gaps between columns of kanji. The notes had to be small and fast to write, so they clipped a single piece out of a kanji and used it as a symbol. These reading notes are called kunten.

Hence "partial kana". Unlike hiragana, which is a whole kanji loosened up, each katakana is a piece of one — few strokes, mostly straight.

Which piece got taken

Pairs that split from one kanji

Often a hiragana and a katakana come from the same kanji — one smoothed, one clipped.

• 宇 → う and ウ
• 幾 → き and キ
• 毛 → も and モ
• 奴 → ぬ and ヌ
• 川 → つ and ツ
• 女 → め and メ

Worth remembering: when a hiragana and its katakana partner look alike, it is not a coincidence — they share a root.

Now it handles loanwords

Until the Meiji era, official documents and laws were written in kanji and katakana. Hiragana only took over ordinary prose later, and katakana was reassigned to loanwords, foreign names, sound effects and emphasis.

So a run of katakana in a Japanese sentence usually means a word from outside. If you cannot place it, read it out loud — if it came from English you will often hear it immediately. テーブル (tēburu) is table; ホテル (hoteru) is hotel.