override

[Jean-marc LEGRAND]

Hi list.

On XP lily2.4.2, I use to define :

tr = \markup {\musicglyph #"scripts-rcomma"}

and make : \once \override TextScript #'extra-offset = #'(2.1 . -1.7) a8^\tr, 
for example, which
puts a comma just after the note (ancoent way to indicate a trill in baroque 
music).

the thing is that I can't understand what the -1.7 respresents exactly. I 
always indicate this
number in a relative way (considering other tr around). But I can't say "for an 
A, the override is :
\override TextScript #'extra-offset = #'(2.1 . -1.7) , for a B :  \override 
TextScript
#'extra-offset = #'(2.1 . -2.5)" , and so on.

Can anyone explain to me what these numbers really mean ?