I have been able to achieve it as following, but the first frame width is
fixed so will cause problems with other chapter numbers such as VIII


\unexpanded\def\HeadTitle#1#2%
{\hbox to \hsize \bgroup
\hfill
\setupframed[offset=.5em,frame=off]
\tbox{\framed[width=22mm,align=right]{#1}}%
\tbox{\framed[width=130mm,align=right,leftframe=on,
rulethickness=0.8pt]{#2}}%
\egroup}

\setuplabeltext[chapter=Boek~]

\setuphead[chapter][style={\bfc}, command=\HeadTitle, textstyle=bold]

...

\startchapter
  [title={Algemene Beginselen},
   ownnumber={I}]

Op di 19 mrt 2024 om 15:49 schreef Hraban Ramm <texml@fiee.net>:

> Hi Jeroen,
>
> it might make sense to use
>
> \setuphead[chapter][
>   conversion=R,
> ]
> \setuplabeltext[nl][chapter={Boek }]
>
> instead of ownnumber.
>
> I would use one command for number and title and put both in the same kind
> of frame. Probably you should use a \strut in the text or
> \inframed[strut=yes] to assure the same text height.
>
> HTH, Hraban
> Am 19.03.24 um 12:59 schrieb Jeroen:
>
> I have some style adjustments for the chapters. The chapter "numbers" need
> to be Boek I, Boek II, etc. A vertical separator was introduced with an
> inframe. The chapter title at the right side does not vertically align
> properly with the chapter number in the frame (inside the frame is
> vertically centered, the title not). Is there a way to align the chapter
> title with the chapter number?
>
>
>
> \starttexdefinition MyNumberChapterCommand #1
>     \inframed[frame=off,rightframe=on,framecolor=black,
>      rulethickness=1px,height=30pt]{#1\enspace}
> \stoptexdefinition
>
> \setuphead[chapter][style={\bfc},numbercommand=\MyNumberChapterCommand,
>     page=no]
>
> \starttext
>
> \startchapter
>   [title={Algemene Beginselen},
>    ownnumber={Boek I}]
>
> \samplefile{tufte}
>
> \stopchapter
>
> \startchapter
>   [title={Organisatorische Structuren},
>    ownnumber={Boek II}]
>
> \samplefile{tufte}
>
> \stopchapter
>
> \stoptext
>
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