A contour line is the shoreline that would appear if the sea rose to exactly that level.
That is the whole idea. Every line follows a fixed height, all the way round. Everything inside the line is higher, everything outside is lower. Once you see that, the rest is easy.
Contour interval: the height between two lines
The contour interval is the most important number on the map and it is almost always printed in the margin. It tells you how many vertical metres there are between two grey lines.
On Lantmäteriet's mountain map at 1:100,000 the interval is 20 metres. In the more detailed product Topografi 50, used at scales down to 1:15,000, it is 10 metres in mountain areas and 5 metres in the rest of the country. Giving the mountains coarser lines is practical: at a five metre interval a steep mountain would turn into a solid black smudge.
If you do not check the interval first, you cannot interpret anything else on the map. Ten lines means 200 metres on one map and 50 on another.
Tight means steep, and you can do the maths
Tightly spaced lines mean steep ground. Everyone knows that. What is less well known is that you can get the gradient in ten seconds.
On a map at 1:100,000 one millimetre corresponds to 100 metres on the ground. If the interval is 20 metres and two lines sit one millimetre apart, you have climbed 20 metres over 100 metres of distance. That is a 20 per cent gradient, about eleven degrees. If they sit half a millimetre apart it is twice as steep.
That calculation is what makes a map useful when planning a day in the mountains, rather than merely beautiful.
Four patterns that cover most of it
Closed rings. A summit. The innermost ring is the highest part. If it is a hollow or a bowl instead, the line carries small ticks on the inside pointing downhill.
V shapes pointing uphill. A valley, usually with a stream in the bottom. The point of the V aims towards higher ground.
V or U shapes pointing downhill. A ridge or spur running out from the mountain. Trails and ski runs often follow these, because a ridge drains and a valley collects water.
Lines that almost merge. Cliffs. On genuinely vertical ground the lines run together entirely and the map stops telling you anything, which is why hill shading is laid over the top.
What contour lines do not show
Contour lines are one of cartography's most elegant inventions, but they have a built-in limitation: they are samples, not a surface. Everything happening between two lines is gone. A nineteen metre cliff sitting exactly between two twenty metre contours does not appear at all.
They also require you to build the model in your head. You see lines and reason your way to a shape. Most people do it moderately well, and almost nobody does it quickly.
That is exactly the problem a physical relief solves. There are no levels to interpret, only a surface with its actual gradient, casting real shadows as the light in the room moves. How much that surface is scaled vertically, and why, we have written about in the article on vertical exaggeration.
To see what a mountain looks like once the lines have become a surface, the whole range sits under mountain pieces.