Merged custom sets allow you to merge several existing custom sets into one merged set. You can select the weights of each subset to control how many problems from each subset will be served. For example you could create three subsets Fork, Pin and Discovered Attack, asking for 70% forks, 20% pins and 10% discovered attacks. Another example would be a merged subset of mate and non-mate sets with say 80% non-mates and 20% mates, you could also merge a larger number of sets, and control mates versus non-mates based on mate length, so for example 80% non-mates, 15% mate in ones, and 5% mate in twos. Problem attempt based sets can also be merged, for example, two sets for problems you have seen before and problems you have not seen before could be merged with say a 90% weight on new problems and 10% on old problems, allowing you to review old problems on average 1 in every 10 attempts. Essentially any existing user created custom sets can be merged (assuming all subsets are of the same problem type, i.e. you cannot merge tactic and endgame sets).

To merge sets you need to go to the manage custom problem sets panel (see Managing Custom Problem Sets section of the manual). You can then select the two, or more problem sets you wish to merge, and then click the Merge button. You will then be prompted to enter a name for the new merged set, ad will need to provide percentage weights for each subset. Weights will need to total 100%.

Once you have chosen your merged set as your current problem set, problems will be served up based on the percentage weights you have provided.