The mutations seem to have started in the second half of the 6th century in the case of the tenues.
The tenues of the classics (as far as they go) are sharp, the medi flat.
As a rule the medial mutation of the tenues and mediae is not denoted in O. Welsh.
It is not until much later times that the h in the mutated forms of the tenues, or the use of the dot, was extended to the mediae.
The same remark holds good in the case of the nasal mutation (eclipse) of the tenues.