List.Accumulate is a very powerful function that is often overlooked, when doing Power Query transformations. Its basic behaviour is explained in the reference. How this function works is accumulates a result from a specified operation (accumulator function) starting from the initial value – seed – and going row by row till the end of a specified list. Radacad greatly explained the possibilities of the function. I treat List.Accumulate as a function that lets me access a previous row. Something that is very often expected by customers using Excel sheets as data sources. This enables me to e.g. detect a change in ID or date – from my experience 90% of the cases.