mgalloy's answer is correct, but you might also see people (like me), who don't use begin/endif when there's only a single line. (of course, this leads to problems if someone goes back and tries to insert a line, not realizing what you did, so Michael's approach is probably better ... this is just so that when you see this formatting, you realize it's doing the same thing:
if (diff lt thres1 && diff gt thres2) then $
print, 'case 1' $
else if (diff gt thres1) then $
print, 'case 2' $
else if (diff lt thres2) then $
print, 'case 3'
or a format that might make someone less prone to insertion:
if (diff lt thres1 && diff gt thres2) then print, 'case 1' $
else if (diff gt thres1) then print, 'case 2' $
else if (diff lt thres2) then print, 'case 3'