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tokens: M 87 is likely anchored close to the innermost stable circular orbit (ISCO). Performing such an analysis in other sources is clearly fundamental for testing the variety of possible scenarios. However, while future instruments with improved capabilities (e.g. phased ALMA) will certainly enrich the sample of suitable targets, this is currently quite limited. In this Letter we present, for the first time, a detailed imaging of the jet base in Cygnus A through global VLBI observations at 86 (GMVA). Among the powerful FR II radio galaxies, Cygnus A is one of the very few two-sided sources whose jet launching region can be probed with sufficient sensitivity at high radio frequencies. At the source redshift () 1 and assuming a black hole mass of ( Tadhunter et al. 2003 ), GMVA observations allow scales of the order of to be probed. The large jet viewing angle enables a prominent counter-jet to be detected in this source, which represents a fundamental advantage when investigating the nuclear regions of a jet. In fact, since the transverse width can be determined continuously along the two-sided flow, the assumption-dependent back-extrapolations of the properties of the jet apex can be replaced by actual measurements. The following analysis complements a previous VLBI study at 43 of the jet transverse profile and of the kinematic properties ( Boccardi et al. 2016 , hereafter B1). This revealed the presence of a pronounced transverse stratification, both in flux density (limb brightening) and bulk Lorentz factor (), typical of a flow with spine-sheath structure. Table 1 Log of observations at 86 GHz (3 mm) and characteristics of the clean maps shown in Fig. 1 . Notes. Column 1: Date of observation. Column 2: Array. VLBA_3mm indicates the part of the Very Long Baseline Array (all but the Hancock and St. Croix antennas) which is equipped with 86 GHz receivers; On – Onsala 20 m; Eb – Effelsberg 100-m; PdB – Plateau de Bure Interferometer; PV – IRAM 30-m atop Pico Veleta; Mh – Metsähovi. Column 3: Beam FWHM and position angle for untapered data with uniform weighting. 2. Observations and VLBI images The GMVA data set comprises three epochs from observations in 2009/2010 with a cadence between five and seven months (Table 1 ). The duration of the observations was 1516 h, 8 of which were spent on Cygnus A. This, together with the large number of antennas employed and with the favourable declination of the source, enabled good uv-coverage. Data were recorded in single polarization mode with a recording rate of 512 , which corresponds to an observing bandwidth of 128 . The correlation took place at the MPIfR in Bonn, Germany. The data reduction was carried out in AIPS ( Greisen 1990 ). Aside from the standard calibration procedures, special care was devoted during the fringe fitting. First, as described in Martí-Vidal et al. (2012) , the manual phase calibration was applied. Single sub-band phases and delays were calculated using one or more scans showing