I have read in this wiki about the alternative trill. However it does not show as a symbol. How do I notate the trill that indicates that a lower note than the one on the sheet should be played in the trill?
When notating ornaments, the "main" note — the one that is part of the melody even if the ornament were removed — must be notated explicitly, with the ornament notation added to it.
The assumption with trill indications is that the trill will involve the main note and the note above.1 A trill from the main note downward should be expressly indicated, as there is no symbol specific to it. This can be done in a variety of ways. I rather like @rfbw's parenthetical note solution, but there are two others which are more common.
- Add a footnote. This allows, either in text or by notation, for the indication of how the trill should be played.
- Add an ossia explicitly notating (some portion of) the trill.
Both of these options are unambiguous and will preserve the intrepretive importance of notating the main note.
1 Unless otherwise indicates, the "note above" will be the next diatonic note. For a chromatic "note above", an accidental is included above the main note (and below any tr indication).
To indicate, that the secondary note is not the upper neighbor in the scale, notate either a two-tone alternating tremolo (as mentioned earlier in a comment by @AndyBonner) or a pitched trill (as suggested in Behind Bars by Elaine Gould, although the term 'pitched trill' seems to stem from the LilyPond community):
Keep in mind, that the tremolo could be interpreted either as a free tremolo or a measured tremolo. The measured tremolo would be an abbreviation for strictly rhythmic notes – 32nd notes (demisemiquavers) in this case. The pitched trill does not imply a strict rhythm.
A trill to a lower note is equivalent to a trill from the lower note to the upper one with a preceding grace note, so that is how it is normally notated.
From the book "Behind Bars" by Elaine Gould (p.138):
A grace note may specify when the starting note is other than the written note
Note: starting a trill from the upper note was the default during the baroque era, and this persisted until well into the classical period.