All the modes adhere to a ternary principle of metre, meaning that each mode would have a number of beat subdivisions divisible by the number 3. Some medieval writers explained this as veneration for the perfection of the Holy Trinity, but it appears that this was an explanation made after the event, rather than a cause (; ). Less speculatively, the flexibility of rhythm possible within the system allows for variety and avoids monotony. Notes could be broken down into shorter units (called "fractio modi" by Anonymous IV) or two rhythmic units of the same mode could be combined into one ("extensio modi") . An alternative term used by Garlandia for both types of alteration was "reduction" . These alterations may be accomplished in several ways: "extensio modi" by the insertion of single (unligated) long notes or a smaller-than-usual ligature; "fractio modi" by the insertion of a larger-than-usual ligature, or by special signs. These were of two types, the "plica" and the "climacus" .