A gap has three features that mark it out from an ordinary bad patch: it is specific - one skill, not the whole subject - it is consistent, showing up the same way in unrelated pieces of work rather than once, and it does not close on its own even when the child is clearly trying. A child who gets a maths topic wrong on Monday and right on Friday is inconsistent, which usually means confusion rather than a gap. A child who gets the same type of sum wrong the same way for three weeks running has a gap.
At Class 1, the commonest real gaps are in matching letters to their sounds, so a child cannot sound out an unfamiliar word and instead guesses from the picture, and in counting back rather than forward, so subtraction goes wrong in a specific, repeatable way. Neither is unusual this early, but both are teachable - sound-first phonics and objects-before-numbers work, in that order, and a class teacher managing a full classroom rarely has the minutes to do either one-to-one.
By Class 4, reading has usually become accurate but comprehension lags behind it - a child reads every word correctly and still cannot say what a paragraph was about, because decoding and understanding are different skills and the second one has to be taught, not caught. In maths, times tables recited as a chant but not usable inside a word problem follow the same pattern: the fact is stored, the application is not.
By Class 8, a gap often hides behind a steady-looking report card, because a child who was comfortable a couple of years earlier can copy notes and pass tests on memorised formulas without ever being asked to apply them - until an exam question is phrased slightly differently and the mark disappears. In Science, this shows up as a child who can define a term perfectly and cannot explain why something happens.
By Class 10, the gap is frequently an earlier foundation that was never solid - factorisation, for instance, learnt in one direction only, which makes quadratic equations painful years later for a reason that has nothing to do with Class 10 itself. This is the single most common thing a diagnostic conversation with a new teacher turns up: not this year’s problem, but an unrepaired one from two or three years back.
The common thread: a gap is a missing building block, and building blocks do not appear because a child got older. This is the kind of problem one-to-one teaching is built for - a teacher who has the time to find exactly where the understanding stopped, rather than a full classroom to get through a syllabus with. It is also why a diagnostic conversation matters more than the label on the class: a Class 8 child struggling with algebra may need forty minutes on Class 6 fractions before algebra makes sense at all, and a good teacher says so rather than pushing on with the syllabus in front of them.
What a diagnostic conversation actually finds
A Class 8 parent once described her son as “bad at algebra” - stuck on every equation, guessing at answers, avoiding maths homework altogether. A first session did not start with algebra at all. The teacher asked him to simplify 3/4 + 1/6 on paper. He could not find a common denominator without being walked through it step by step, and that single fraction sum explained the entire pattern: every algebra question that needed combining unlike terms was quietly failing two years earlier than anyone had noticed. Three weeks rebuilding fractions, and the same algebra questions that had looked impossible in September started resolving themselves by November - not because algebra got easier, but because the missing floor underneath it finally got built.