Lesson 9 of 11
The big picture
Show on a big screenThe score sheet, bangle packets and date plate each write numbers differently.
| Idea | How it goes |
|---|---|
| One mark each | One stroke per thing. Exact, but slow to read. |
| Bundles | A gap every few marks. 23 is 4 bundles of 5, plus 3. |
| Symbols added | Symbols add. XXXVII is 37. |
| The pair rule | Six pairs take away: IV, IX, XL, XC, CD and CM. IV is 4, IX is 9. |
| The column | Our symbols have no fixed worth — a 5 is 5, 50 or 500 by column. |
| The empty place | With no mark for it, 205, 25 and 250 all write as a 2 then a 5. |
| Ten symbols | Ten marks, a position rule and a zero reach every whole number. |
| Cost of adding | Columns cost about a mark per digit. Symbols cost at least three times as many. 27 + 15 is 7 marks, then 24. |
Part I, Chapter 3 is A Story of Numbers. Read the chapter and see which of these ideas it uses.
One question covers it all: symbol worth, or place worth? A symbol keeps its own worth. A digit's worth depends on its column.