When a cake tin is specified at 18cm, is this the depth or the width? Would I be right in thinking that a certain depth gives a specific width?
There are two main types of round cake tins/pans but when a size is specified it is always the diameter (width) of the tin, and not the depth. The most common sizes of tin are 20cm/8 inches and 23cm/9 inches, though you can get smaller and larger sizes and an 18cm/7 inch tin is not uncommon.
The two types are the regular cake pan or sandwich tin which is used to make a two later cake (these tins may have loose bases or fixed bases, depending on where you live) and also the springform tin which is much deeper and has a clip on the side which can be used to help unmould the cake or cheesecake. The springform pans always have loose bases. In the UK you can also get extra deep cake tins (often used for Christmas Cakes) with loose bases, and these usually have the same dimensions as a springform pan.
The depth of sandwich tins can vary slightly but they are roughly 4cm/1 10/16 inches deep, regardless of the diameter. Springform pans and deep cake tins are usually 8cm/3 1/8 inches deep, again regardless of the diameter.