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We explore the structures of protoclusters and their relationship with highredshift clusters using the Millennium Simulation combined with a semi-analyticmodel. We find that protoclusters are very extended, with 90 per cent of theirmass spread across $\sim35\,h^{-1}{\rm Mpc}$ comoving at $z=2$ ($\sim30\,\rm{arcmin}$). The `main halo', which can manifest as a high redshift clusteror group, is only a minor feature of the protocluster, containing less than 20per cent of all protocluster galaxies at $z=2$. Furthermore, many protoclustersdo not contain a main halo that is massive enough to be identified as a highredshift cluster. Protoclusters exist in a range of evolutionary states at highredshift, independent of the mass they will evolve to at $z=0$. We show thatthe evolutionary state of a protocluster can be approximated by the mass ratioof the first and second most massive haloes within the protocluster, and the$z=0$ mass of a protocluster can be estimated to within 0.2 dex accuracy ifboth the mass of the main halo and the evolutionary state is known. We alsoinvestigate the biases introduced by only observing star-forming protoclustermembers within small fields. The star formation rate required for line-emittinggalaxies to be detected is typically high, which leads to the artificial lossof low mass galaxies from the protocluster sample. This effect is stronger forobservations of the centre of the protocluster, where the quenched galaxyfraction is higher. This loss of low mass galaxies, relative to the field,distorts the size of the galaxy overdensity, which in turn can contribute toerrors in predicting the $z=0$ evolved mass.

What are Protoclusters? -- Defining High Redshift Galaxy Clusters and Protoclusters