Note and dotted note on the same line - Lilypond

Question

I have a note here:

What do these notes (a quarter note and dotted quarter note on the same line) mean?

It's note a, in LilyPond, bottom of the bass clef.

How is this coded in LilyPond?

Full measure score here:

Answer 1

It means there are two "voices": one of them "sings" a dotted quarter note on a certain pitch, the other sings a series of sixteenth notes beginning on that same pitch.

More commonly, you would see this written with a double stem, as in the upper staff. However, in this case, because of the "dot", it must be written as a double note. If not, it would seem as though the upper voice of the two was responsible for a dotted sixteenth note.

This is not a LilyPond issue; it's a standard notational convention.

For instructions on coding this in LilyPond, see the Collision resolution section of the LilyPond documentation.

Here is a code snippet based on the documentation.

\score{
<<
  {
    d'16
  } \\ {
    d'4.
  }
>>
}

...which produces

Answer 2

In addition to Aaron's answer, you can also overlap these two noteheads if you like by introducing the \mergeDifferentlyDottedOn command:

The code to create this is:

\version "2.20.0"

global = {
  \key c \major
  \time 6/8
}

music = \relative c {
  \global
  \mergeDifferentlyDottedOn
  <<
    {
      a16\( e' a e a e\)
    }
    \\
    {
      a,4.\sustainOn
    }
  >>
}

\score {
  \new Staff { \clef bass \music }
  \layout { }
}