In June 2000 Steve Ekwall proposed the theory that there was a 'folding key' system to interpreting the Voynich Manuscript, using the gallows characters.

More information on the Ciphermysteries page [1] with a discussion, and links to a film and a PDF on the subject etc. YouTube link here [2].

The main 'arguments against' the idea are the length and neatness of the manuscript - the time involved in incorporating the key into such a long text, and the absence of other texts using the same process.