What's the difference between diatonic and melodeon?

Diatonic

Definition:

(a.) Pertaining to the scale of eight tones, the eighth of which is the octave of the first.

Example Sentences:

(1) In Experiment 1, children from 4 to 6 years of age were superior in detecting the semitone change in the diatonic context compared with the nondiatonic context.
(2) The factors were distance in pitch height from the context tones, octave equivalence, and the following hierarchy of tonal functions: tonic tone, other tones of the major triad chord, other tones of a diatonic scale, and the nondiatonic tones.
(3) In Experiment 2, infants 9 to 11 months of age detected the semitone change in all positions, but their performance was not influenced by diatonic context.
(4) The sequence consisted of a series of frequent standard C5 tones from the diatonic musical scale, in which three deviants (F #5, C6, and F #6) and a target (C4) were occasionally embedded.
(5) They were tested for their detection of two types of changes to that melody: (a) a 4-semitone change in 1 note that remained within the key and implied dominant harmony (diatonic change) or (b) a 1-semitone change in the same note that went outside the key (nondiatonic change).
(6) Adults easily detected the nondiatonic change but had difficulty with the diatonic change.
(7) These findings indicate that infants and children can discriminate a semitone in a musical context and that the priority of diatonic structure emerges by 4 to 6 years of age.
(8) In the present research we examined the development of sensitivity to two musical relations significant in Western tonal music, the semitone and diatonic structure.
(9) Two standard melodies were used, one composed of diatonic tones only and the other containing a non-diatonic tone.
(10) Analysis of responses to melodies in each of the 24 major and minor keys indicated sensitivity to aspects of diatonic structure exhibited by mature listeners.
(11) Listeners rated test tones falling in the octave range from middle to high C according to how well each completed a diatonic C major scale played in an adjacent octave just before the final test tone.