Thanks Dave I was not clear enough, -Head can have values; both, all, begin, end, whereas Q can be 0, 1, 2, 3, so some go-between is required. I finally looked and found part of it in source file thline.h // tags pre arrow TT_LINE_TAG_HEAD_BEGIN = 1, TT_LINE_TAG_HEAD_END = 2, Presumably somewhere else NONE is set to or implied as zero and BOTH is set to BEGIN + END. That would complete the mapping from the data type that -Head takes as input in a th2 file, to the 0, 1, 2 or 3 that ‘line arrow’ requires for variable Q. I rather suspect I don’t have the requisite knowledge for this, but sometimes I feel the need to push the boundaries anyway! :) Bruce From: Therion <therion-bounces@speleo.sk> On Behalf Of Dave Clucas via Therion Sent: Saturday, 31 March 2018 2:28 PM To: therion@speleo.sk Cc: Dave Clucas <dave.clucas@icloud.com> Subject: Re: [Therion] scaling line ornamentation - metapost (Bruce Mutton) I’m not sure about this but I suspect that it doesn’t involve parsing. It’s likely that the metapost expects a parameter or parameters to be passed to it which it then assigns to the variable Q in its own procedure. It won’t care what the calling program calls it. I think the arrowhead is probably defined within an arrow class (arrow.h perhaps). On 30 Mar 2018, at 18:00, therion-request@speleo.sk <mailto:therion-request@speleo.sk> wrote: I notice line arrow takes the special argument -head [both | none | begin]. Somewhere between the th2 file and the metapost arrow definition, the argument -head gets changed to the variable Q that is parsed to the metapost code, that controls which ends of the arrow line get arrow heads. I have copied the code below. Whereabouts is the code that tells Therion that -head is parsed to the metapost definition as Q? Dave Clucas <mailto:daveclucas@icloud.com> dave.clucas@icloud.com Exploring the World - One cave at a time