Rationale behind buying 51%, 100% or sth in between

51% allows for majority control

I don't really know, but I know that if you only own 51% there is still a board of directors that controls the CEO (who controls the company). If you only own 51% you can control who makes it onto the board of directors, but you cannot really control their day to day actions (unless you managed to get a bunch of yes men). If you own 100% of a companies shares (or in some states 85-90% and squeeze out the remaining 10-15%) you no longer need a board of directors, and you are now the companies only ruler. I imagine financing could be part of it, as well as exposure. Maybe some companies are a little to risky to dump the money in for 100%, but you can still control the company at 51%.

I don't really know if that answered your question, but there you go.

thank u, there are some great thoughts... are there more reasons than effectiveness of control, financing and exposure?

100% essentially takes the firm private.

Probably more company specific situations, but reasons could be related to how much you can afford to buy, how much you need to buy for control or other motive (e.g., getting one member on the board vs. many more, selling off business units easily), how many shares are even available to buy (might be other large holders), or who you can convince to sell you that many shares (e.g., a large shareholder may only want to sell you some, but not all). Also, in some situations, you can exercise majority control with a less than 50% stake.