This page has a few examples of some of the ways the numbering system can be put together and how it would be read.
If you were adopted and are tracing both your natural and adopted trees, you would start your adopted tree with 1a or 2a. If you were female your adoptive mother would be 2a2. If your direct ancestor was adopted, and you know their natural parents, the natural line would continue on as such 1221 and 1222. The adoptive line would then go 122a1 and 122a2.
If you are tracing back from someone who is not a direct ancestor, the period is used to go back up and indicate another line. 121.34a would be my step-cousin. This persons adoptive parents would be 121.3 and 121.3.s, or my Aunt and Uncle. This persons natural parents would be 121.34a.1 and 121.34a.2 .
In digging up your tree, you may find ancestors who had more than one spouse at a time. If there were many children involved it may be easier to number the children as children of the respective spouses.
2211211 had ten wives. They would be numbered 2211211.1s to 2211211.10s . Of course if you were descended from his 5th wife there would be no 2211211.5s, she would be 2211212. The children of the other wives would be in this form; 2211211.4s3, 2211211.4s4 ect. The parents of this spouse would be 2211211.4s.1 and 2211211.4s.2.
You must always be aware of the direction you are going. If it was a cousin that had many wives it would take this form: 121.32 is the cousin, 121.32.1s to 121.32.5s would be the spouses. The children would then be 121.32.4s.3 and 121.32.4s.4 and the spouse's parents would be 121.32.4s1 and 121.32.4s2 . It is not that confusing if you keep in mind the direction changes that the periods indicate.
Here is the potential to really have lots of direction changes. Lets use an involved example. Let's say my uncle married three times. Each time he had three children. The second and third wives were previously married and had three children by their former husbands. He is still married to aunt#3. How do we number everyone?
uncle = 121.4 aunt1 = 121.4.1x children of = 121.4.1x.3 aunt2 = 121.4.2x spouse1 = 121.4.2x.1x children of = 121.4.2x.3 children of = 121.4.2x.1x3 aunt3 = 121.4.s spouse1 = 121.4.s.x children of = 121.43 children of = 121.4.s.x3
As you can see, the period not only changes the direction along the tree, but when used around spouses also slides you sideways.
I am sure I have not covered all the possibilities. If you have a problem or a question, please email me and I will rack my brains to find you an answer.