Any positive matrix Mpartitioned in four n-by-n blocks satisfies the unitarily invariant norm inequality parallel to M parallel to <= parallel to M1,1 + M2,2 + omega I parallel to, where omega is the width of the numerical range of M1,2. Some related inequalities and a reverse Lidskii majorization are given. (C) 2017 Academie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license.