Regulatory and prohibitory signs
Regulatory signs are CIRCULAR, and they carry the force of law. A red border means a prohibition; a blue circle means a mandatory instruction.
Prohibitions include no entry, no parking, no stopping, no overtaking, no horn, no U-turn, no right or left turn, and the weight, height, width and length limits.
Mandatory blue signs tell you what you MUST do: go straight, turn left, keep left, or a compulsory roundabout.
Two are shaped apart on purpose so they are recognisable even when unreadable: STOP is octagonal, and GIVE WAY is an inverted triangle.