How many letters does the top answer for “to pass from one key into another.” have?

The most common answer is modulate (8 letters).

Crossword clue: to pass from one key into another.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

modulate 8 letters web-def

vary the frequency, amplitude, phase, or other characteristic of (electromagnetic waves)

another word for key 50% similar → cardinal, fundamental

another word for one 50% similar → ace

another word for pass 50% similar → walk