Multi-tap presses a key once per letter position. T9 uses one digit per letter.
Before touchscreens, phones had twelve keys and the letters were printed on 2 to 9: ABC on 2, DEF on 3, up to WXYZ on 9. There were two ways to type with them, and puzzles use both.
Press a key once for its first letter, twice for the second, three times for the third. H is 4 pressed twice: 44. HELLO is 44 33 555 555 666, and 0 is a space. It's unambiguous as long as the presses are grouped, so the decoder needs the spaces between letters. If they're missing, a long run like 555555 is read as two full presses of L.
T9 (“text on 9 keys”) took one press per letter and let a dictionary work out the word: 43556 could only really be HELLO. Some digit strings fit several words, the famous one being 4663: GOOD, HOME, GONE, HOOD. The decoder checks every word in a 20,000-word dictionary and lists the most common match first, with the others underneath. Click one to swap it in.
7777 is S, and only 7 and 9 go to four).Multi-tap is how old phones typed: press 4 once for G, twice for H, three times for I. T9 uses one press per letter and lets a dictionary pick the word, so 4663 could be GOOD, HOME or GONE.
With 0, as on a real keypad. In multi-tap, spaces between the digit groups separate letters, so 44 33 is H E.
Every word in a 20,000-word dictionary is turned into its keypad digits. Your digits are matched against them, and the most common word comes first, with alternatives below.