If you're out and about and you see something you think will work for your modelling scale...but you've forgotten your ruler,
Here's a site that automatically detects your Phone or Tablet Screen Size and gives you an image of a ruler that can be used in Centimeters or Inches
Found it somewhat by accident. I needed to measure a little piece of hardware that could be used for an Axlebox
And if you don't always have Internet Service available, you can do a screenshot of the ruler and save it !!!