The great thing about those keyboards was that a lot of people could just turn off text prediction and type text with a single hand without looking at the screen.
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The great thing about T9 (certainly on the Nokia 3210 and 8210) back in the day was you could type messages fast with few k/s without looking. As long as you had enough experience to know the word combos.
Ive never found a t9 system as good as the Nokia implementation. In some respects its better than qwerty for short messages. And don’t get me started on apples fundamentally broken auto correct system. People dont know any better these days. There’s actual adults walking today that have never typed on a real keyboard.
On the nokia you'd press the button and it would cycle through the options. Once you knew all the words you could type really very fast indeed, and blind.
Someone asked about custom words below. You could definitely add custom words. I think you had to switch out of T9, key the word the old way and then switch back then 'add' to dictionary, but once in it would stay in the dictionary. I'm sure the amount of memory for custom words was quoted in marketing material at somepoint.