  We summarize the current top-quark mass measurements from the CDF and DO
experiments at Fermilab. We combine published Run I (1992--1996) measurements
with the most precise published and preliminary Run II (2001--2011)
measurements based on data sets corresponding to up to 8.7 fb^{-1} of ppbar
collisions. Taking correlations of uncertainties into account, and combining
the statistical and systematic uncertainties, the resulting preliminary
Tevatron average mass of the top quark is M_top = 173.20 \pm 0.87 GeV/c^2,
corresponding to a relative precision of 0.50 %.
