How to schedule singleton courses
A singleton is a course with only one section, so it meets in just one period. To schedule singletons, list them first, count which students request two or more of them, and spread those pairs across different periods. Then build the rest of the schedule around them and check which requests still miss, and why.
Updated October 2026
What is a singleton course?
A singleton course is a class that runs as a single section, with one teacher in one period. Every student who wants it has to be free in that period.
Most schools have more of them than they expect. Typical singletons include:
- AP and other advanced courses with one teacher qualified to teach them
- Upper levels of a world language
- Band, orchestra and choir
- Specialty electives that only fill one class
A course with two sections, often called a doubleton, behaves much the same way. It gives a student two periods to fit it into instead of one.
Why do singletons cause so many conflicts?
A student who wants two singletons that land in the same period can only take one of them. There is no other section to move them to.
A course with several sections is forgiving, because the build can shift a student to another period. A singleton has nowhere else to go, and each one you place narrows the choices for the rest. That is why small schools, where many courses run once, often have the harder problem. See scheduling for small schools.
How do you find which singletons conflict?
Count how many students requested each pair of singletons. The pairs with the most shared students are the ones that must not share a period.
Schools traditionally do this with a conflict matrix: a grid with every singleton down the side and across the top, where each cell holds the number of students who asked for both. Include doubletons too. Do it after course requests close, because the counts change as students change their minds.
What order should singletons be placed in?
Place singletons first, starting with the ones that share the most students with other singletons. Courses with many sections go last, because they can fit around almost anything.
- List every course with one section, and those with two.
- Note the fixed limits: part-time teachers, shared rooms such as a lab or gym, and periods a course must avoid.
- Give a teacher's singletons different periods, since one teacher can't be in two rooms.
- Count the shared requests between each pair of singletons.
- Put the most-shared pairs in different periods.
- Place the multi-section courses around them.
- Check which requests missed and why, then adjust and try again.
By hand, the last step is where the time goes. Freeing one stuck student often means moving two or three others first. The steps to build a master schedule cover where this fits in the full process.
How does nOS handle singletons?
In nOS, a singleton is a rule you write as a sentence, such as “Keep AP Biology a singleton in period 3.” Say “must” and it is treated as a requirement; say “prefer” and it gives way only to place more requests.
The build places the hardest things first: the student with the fewest options left, and the course with the fewest places it can go. When a student can only be placed if someone else moves first, nOS makes the whole chain of swaps at once.
Afterwards, every request it could not place comes with a reason in plain words, such as “Conflicts with another class” or “All classes are full.” It does not promise a schedule with no conflicts. It checks the result and tells you what it finds. How nOS builds a schedule explains each pass.
A rule saves now. Any students it moves wait for Apply changes.
No rules yet.
Kept for the 1 class it covers
Kept for all 3 classes it covers
Questions about singleton courses
What is the difference between a singleton and a doubleton?
A singleton has one section and a doubleton has two. A doubleton gives a student two periods to fit the course into, but it still collides when both of its sections share periods with other singletons the same students want.
Should a singleton be locked to a period?
Only when it truly has to be there, for example because the teacher is part-time or the room is shared. Every locked course gives the build less room to place everyone else. In nOS you write it as a rule, such as “Keep AP Biology a singleton in period 3.”
How many singletons is too many?
There is no fixed number. What matters is how many students request several singletons at once, and whether those pairs can all go in different periods. A school with many singletons and little overlap between them can schedule well.
Why do small schools have more singletons?
A small school often has one teacher per subject and only enough students for one class of each elective. A large school can open a second section; a small school usually can’t, so the schedule has to be arranged around the singletons.
Try it with your own singletons.
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