Differences between luk forms

[Greybeard]

Dear fellow members

Despite my nickname -- Greybeard -- I am a "young" keris collector and a new member, too. Still lacking in knowledge and experience, I collected quite a number of older and a few new kerises (I hope they are of good or at least of average quality ... anyway, so far I like them all!).
As a starting point, I decided to purchase kerises luk only, because I am especially interested in the forms of the luk (kemba -- sedeng -- rengkol, according to the Ensiklopedi Keris). However, when looking at my kerises I find it sometimes difficult to distinguish one luk type from the other. E.g. where does sedeng end, where does rengkol begin?
Can someone provide some pictures, showing those styles and the differences between them?

Thanks a lot and best regards,

Greybeard