(a) Sag curve is provided:
(i) When an upgrade is followed by a downgrade.
(ii) When a milder downgrade is followed by a steeper downgrade.
(iii) When a steeper upgrade is followed by a milder upgrade.
(iv) When a steeper downgrade is followed by a milder downgrade.
(b) Summit curve is provided:
(i) When a steeper downgrade is followed by a milder downgrade..
(ii) When a downgrade is followed by an upgrade.
(iii) When a milder downgrade is followed by a steeper downgrade
(iv) When a milder upgrade is followed by a steeper upgrade.