A printer has first and second print sections 100, 200 joined along a line 12, and a controller 50. Each section contains a printhead 120; 220 travelling on a track 110; 210. A frame or frame part 104 supports the track and is driven on wheels by a motor, so that the printer can travel autonomously along a horizontally extending step edge, printing as it goes. It further includes a sensor for detecting the proximity of the vertical face 5v of the step. The printer can thus be set to print advertising, for instance, on the steps of a stadium.