  Since the early days of QCD, it has been argued that inside hadrons quarks
organise into substructures. Among those, popular pictures of various phenomena
(including exotica and colour superconductivity) give a prominent role to
diquarks, i.e. to colour antitriplet combinations of two quarks. Using a
gauge-invariant setup, which combines the diquark with a static quark, we study
spatial correlations of the two light quarks inside the diquark. After
illustrating the setup, we discuss our first results for both the scalar
(``good'') and the spin one (``bad'') diquark channels. In a regime in which
$m_{\pi} \simeq 830$ MeV, our data show unambiguously that a scalar diquark
forms with a size of $\simeq$ 0.9 fm. For the vector diquark we are able to put
a lower bound of 4.1(7) fm on its size. We also investigate the mass splitting
between the good and the bad diquark; our findings are compatible with
phenomenologically inspired predictions for this quantity.
