I've not yet read the other replies.
I said that options 1 & 2 apply.
Maddison had to be PTS due to fibrosarcoma.
Del had to be PTS because old age caught up with him and he was struggling to go on.
Jessie was PTS for the same reasons as Del.
Flo was PTS due to heart failure.
Stanley was PTS due to arthritis making it impossible to go on.
Sally was PTS due to the same reasons as Del & Jessie.
Certainly, when there is a definate ailment involved, the decision can be more definitive but in all honesty, the decision is still never clear cut (with the exception of Flo, with her heart failure). With ALL the others mentioned above, they could have lived another day or another week or maybe longer, but to allow them to have done so would have made them struggle in one way or another, either physically, mentally or both. And I don't want their last days/weeks on earth to be terrible ones - I want them to be as good as they can be, given the circumstances.
There has come a time with each one of the dogs above where watching them try their hardest to continue has become like a physical pain for both myself and DH and we've made the call. There may be outsiders who have looked in and thought that the call was made too early but I don't give two *beeeeps* for what anyone else thinks. They are OUR dogs and we'll make the call when WE feel the time is right. (I don't mean outsiders here on GT, I just mean in general).
The upshot for me is that on the day concerned, for each of the dogs, I have known that the day had arrived. I just pray that I continue to know at the right time and never allow a dog to go on longer, for my own benefit, rather than theirs.
(I'll read everyone elses replies when I get back from the Red Cross. It was not going to be a good idea to read them before I have to leave the house)