Footnotes, references & resources
You will often find notes (tooltips) inside the content. They contain more info about not directly relevant but interesting stuff. Point at them with your cursor to see it.
Every time claims are made or facts are given, the references and sources are shown. All references on a page are listed at the bottom of the page. These references will if possible contain a direct link to a free and accessible location where you can have a look yourself or look up more information.
Once in a while, you’ll see links to extra resources. They are open, freely available pages on this site that contain specific overviews, data collections, tables for look-up etc. They may be interesting and useful for your insight into certain skills.
Also, often when formulas and the like are given, they are followed by links to the proof on this site. Proofs will convince you that the formula indeed is correct, if you wish to dive into the details. Some proofs are simple, others are hardcore; some are intuitive, others are purely mathematical. We have tried our best to make them understandable for the layperson. Note that many proofs are not strictly mathematically accurate or complete – often we rather put emphasis on the intuitive idea of how the formula has come to be.