A spatial sequence synesthete might experience the months of the year as an oval tilting away to the left, or the number line as a path that bends at 20 and climbs after 100. The layout is specific, involuntary, and stable for life.
Many report practical effects: dates are easy to compare because they sit in visible positions, and mental arithmetic can involve literally moving along the line. Francis Galton documented number forms of this kind in the 1880s, making it one of the first forms of synesthesia in the scientific record.
It overlaps with time-space synesthesia, where units of time specifically take spatial form.