A contour is an artificial line that connects points of equal height above the height datum, which is currently mean sea level, as recorded at Malin Head. In the extract from the 1:50 000 Discovery Series below, we see contours at 10 metre intervals, with spot heights at prominent points also given; Meendoran Hill reaches a height of 155m, while Ballintlieve Hill stands at 227m.
Contours tell us a lot about the gradient; note how the contours at Meendoran Hill are spaced widely, in comparison to Ballintlieve Hill. This indicates that the latter is much more steep than the former. Also, notice how the stream flowing north east from Ballintlieve Hill into Lough Fad has eroded into the terrain, creating V-shaped indentations in the contour lines. Spot heights are also not just for land; see how the surface water of Lough Fad is recorded at 125m.
While several contours can merge into one line, they can never cross over each other.
Interpretation of contours is helped by the use of shading, such as shades of green and brown, to give an impression of different heights.