In projective geometry there is a duality principle between points and planes, i.e. every true statement yields a dual statement obtained by interchanging the the words "point" and "plane".

I am looking for a simple way of writing the corresponding statements side by side in two columns without having to align them manually; now and then there occur self-dual statements, which should therefore span the width of the page (not be columnized). I also want to enumerate the statements (one item for each pair of statements).

Here an example:

            \item \label{itm: P1} There are two lines, which have no point and no plane in common
            \item \label{itm: P2} In every line lie at least three points and three planes.
            \item \label{itm: P3} Two distinct planes have one and only one line in common.
            \item \label{itm: P3'} Two distinct points have one and only one line in common.
            \item \label{itm: P4} Two lines have a point in common if and only if they have a plane in common.

I want P3 and P3' to be placed side by side. I want to continue the list with interruptions for quite a bit, which is why I would prefer working with cross-references to doing it manually...

I would be grateful for suggestions!

closed as unclear what you're asking by Andrew Swann, Stefan Pinnow, CarLaTeX, TeXnician, Ruben Sep 12 '17 at 12:51

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    Do you mean something like what the tasks package provide? – user31729 Aug 4 '16 at 7:57
  • Yes, but I would like to have only one item for each double-statement, or what would be even better: the same item for both parts of a double-statement (e.g. a number) but an additional item to distinguish the two parts (e.g. "a)" on the left and "b)" on the right). – Larry Aug 4 '16 at 8:21
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    The \task! command is used for an item that spans all columns. For the ‘better’ way, are you looking for a macro which swaps ‘point’ and ‘plane’? – Bernard Aug 4 '16 at 11:33
  • I still have problems with referencing to a task... is that possible in tasks? I have added an example for what I am looking for. – Larry Aug 4 '16 at 21:28
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    Put P3 and P3' in a multicols environment – Peter Flynn Aug 4 '16 at 21:48