The Latin word insula (literally meaning "island", plural insulae) was used in Roman cities to mean either a city block in a city plan, i.e. a building area surrounded by four streets, or, later, a type of apartment building that occupied such a city block. Insulae were known to be prone to fire and rife with disease. A standard Roman city plan was based on a grid of orthogonal (laid out on right angles) streets. It was founded on the ancient Greek model of Hippodamus. It was used especially when new cities were established, e.g. in Roman coloniae.