If you are lucky enough to have none of the above and have a lovely flat nail like this one then please cut it straight across and it will stay lovely. 

 

Unfortunately, many of us do not have flat nails and have variations on the top images to a greater or lesser degree. Cutting these can be a minefield and really, it's worth seeing a podiatrist and having a lesson on nail cutting. The curly nails, called involuted nails, impact the skin and can cause hard skin and corns to develop along the nail borders. They can become painful.