Layout of the CCDs in Gaia’s focal plane. The star images move from right to left in the diagram. The along-scan (AL) and across-scan (AC) directions are indicated in the top left corner. The skymappers (SM1, SM2) provide source image detection, two-dimensional position estimation and field-of-view discrimination. The astrometric field (AF1–AF9) provides accurate AL measurements and (sometimes) AC positions. Additional CCDs used in the blue and red photometers (BP, RP), the radial-velocity spectrometer (RVS), for wavefront sensing (WFS) and basic-angle monitoring (BAM) are not further described in this paper. One of the rows (AF3) illustrates the system for labelling individual CCDs (AF3_1, etc.). The nominal paths of two star images, one in the preceding field of view (PFoV) and one in the following field of view (FFoV) are indicated. For purely technical reasons the origin of the field angles (η,ζ) corresponds to different physical locations on the CCDs in the two fields. The physical size of each CCD is 45 × 59 mm².