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The side subtending the right angle: the sides containing the right angle. How much can be obtained by ignoring the figure, the diagram, except in so far as it serves a function – like that of the material of a sculpture by Henry Moore – in framing the place where there is no material? To act as a boundary to the open space, that is to say the part where the figure is not.
Then the squares on the sides containing, and the squares on the side subtending, the right angle serve to enclose the triangle – the ‘three-kneed thing’, but also the right angle.
The construction is a trap for light.
Euclid I.5 marks the point at which the ‘elements’ of geometry are left behind when the student crosses the Pons.