Dealing with pocket change.
Adding numbers that have parts less than one.
Simon Stevin introduced the decimal system to Europe in 1585.
Button up your shirt: Keep the decimal buttons (points) in a straight vertical line.
Plain English: Adding decimals is exactly like adding big numbers, but you MUST line up the dots. If one number has no dot, it goes at the end.
Real-world example: Adding $10 (no cents) and $2.50. You must add the 2 to the 0, not the 1.
Lining up the last number instead of the dot.