Information Technology Reference
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poses (e.g. document pages, folders and index stickers). The same core interaction
performed on a different type of page area leads to another result. For example, if
the core interaction of inking is performed on an adhesive bookmarking sticker, it
might define the label of an index, whereas inking on a document page might create
a free-form annotation.
Spatial multiplexing results in a set of specialized tools made of paper. These
tools have the characteristic that they are not only instruments but they can also be-
come objects of interest. Tools that have a purely instrumental function are only
used to manipulate objects of interest (e.g. documents). Examples of traditional
instrumental tools are hammers, scissors and pens. Most tools in Graphical User
Interfaces belong to this category. In contrast, we aim at designing tools that in
addition to their instrumental function are first-class objects. For example, an ad-
hesive sticker that can be attached to a document page in order to bookmark this
page is initially an instrument for creating an index on a document page. But once
the sticker is attached to a document page, it becomes also an object of interest, as
it represents the actual index. Similarly, physical folders are used as an instrument
to define collections of documents. In addition it is itself an object of interest. Its
physical state indicates for example if only a few or many documents are included
in this collection.
Gestural Multiplexing
Different words or gestures written with the same pen on
the same type of page area serve different purposes. For example the keyword “im-
portant” might serve the purpose of a tag while the term “cf. page 3” might serve
the purpose of a link. Gestural multiplexing is very powerful, as a large number of
gestures can be defined. Moreover, it does not require other tools than one pen and
unspecific sheets of paper. However, a heavy use of gestural multiplexing runs the
risk to create a command-based interface which requires that the user memorize a
large number of commands and results in poor usability (see Norman & Nielsen's
recent criticism of gestural interfaces [106]). A heavy use of gestural multiplexing
does not account for the rich practice of traditional interaction with paper docu-
ments, which does comprise tools other than a pen, such as index stickers, page
markers and folders. For this reason, we argue that Pen-and-Paper User Interfaces
should use only a small set of simple gestures, such as points and lines, and put the
emphasis rather on a rich variety of paper tools (and hence on spatial multiplexing).
Device Multiplexing
If performed with another device, the same actions can have
a different signification. For example, inking with a digital pen which has a ballpoint
tip might be used for making textual annotations, whereas inking with another type
of digital pen which features a highlighter tip might be used for marking up pas-
sages. Device multiplexing is powerful if the number of devices keeps manageable
and if the repartition of functions across devices is clear. However, there is empirical
evidence that people tend to use one single pen rather than switching between many
tools [91]. As an alternative to switching between devices, the pen device can fea-
ture physical input elements that allow for switching between modes on one single
pen.
