CV indexing enables us to organize and store related programming functionality under one “Master” CV. The confusing part to many modelers is the fact that access to a Master CV’s programmability can only be gained through its indexes. A Master CV can have as many as two such indexes. Referred to as “Primary” and “Secondary” indexes, they are expressed as decimal places in an indexed CV’s formula. Happily, the CV numbers of the Primary and Secondary indexes remain the same, regardless of which Master CV we want to program. Noting the following “rules” should help:
Rule 1: The Master CV contains all broadly-related functions. Master CV 55, for example, contains all lighting information and functionality.
Rule 2: In any indexed CV, only the Master CV can be programmed to control a given function.
Rule 3: The same two Primary and Secondary indexes – CV 49 and CV 50 respectively – are always used to provide access to a Master CV, regardless of which Master CV is to be programmed.
Rule 4: The values programmed into the Primary and Secondary indexes determine what function will be programmable in the Master CV. Understanding the numbers.
For example, lets use CV55 to illustrate an indexed CV formula expressed as: CV55.76.10.
The 1st  is the master
The 2nd  is the primary index value
The 3rd  is the secondary index value
Another understandable point of confusion for many modelers is the fact that in order to program Master CV55, you have to first enter the decimal values shown in its formula into its Primary and Secondary indexes – CV 49 and CV50 – and do so in reverse order, from right to left, when the formula is read right to left. In the above example then, we’ll first go to CV50 (the Secondary Index) and enter a value of “10,” followed by entering “76” into CV49 (the Primary Index). Once we’ve done that, we can go to CV55, our Master CV, and program its values to whatever level we prefer – which in this case is between 0-255 (its default value is 32). Many people find it helpful to write out a small table to help keep the values straight.
CV# CV50 ……. CV49……..CV55
And there are two more Rules:
Rule 5: Not all indexed CVs require entry of a Secondary Index value. If a formula shows only one decimal place, as does the 51.0 formula of the Master System Volume, for example, the Secondary Index value is disregarded. In this example, we’d go directly to CV 49, our Primary Index, and enter “0.” We’d then go to CV51 — and have access to its 0-127 volume control programmability.
Rule 6: The best news: you only have to enter values into the Primary and Secondary Indexes the first time you program a Master CV.