A notation does not belong to all mathematicians in the same way. Each field, sometimes each school, has its shortcuts, and outsiders trip over them. This is not a flaw: it is what happens with any living language, which splits into accents and jargons. But it has consequences for readers and for learners. Einstein erases the summation sign In 1916, in the paper that presented general relativity, Albert Einstein had to write hundreds of expressions like