When a tree stump has been exposed you can see the rings. It is thus possible to tell how old the tree was. You can also see the wide rings which are caused by good growing conditions because the summer weather was very good. Summers with less growth show narrower rings. This pattern of narrow and wide rings is very unique, it’s almost like a fingerprint.
The patterns from the trees or from things made of wood that aren’t the same age but have grown together for a time can be placed side by side (see drawing). Now it’s like a puzzle. The patterns join and can go back in time if one has found usable items of wood from different time frames. If you know the age of the items you can then write the years on them. If you find a piece of wood and don’t know its age you can compare the pattern with the ones you have already and calculate the age on that basis.