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Here's a mnemonic aid sent in (9/00) by Barbara D. Martin for the relationship between different metric lengths, and teaching how far to move the decimal point, and in what direction, when converting lengths within the metric system:

Killer Hound Dog Meets Deadly Cat Man

In the metric system of measurements, the principal unit for length is the meter; the principal unit for capacity the liter; and the principal unit for weight the gram. To convert from one unit of length to another just move the decimal point as many places to the left or right as indicated in the mnemonic sentence/chart. For example:

Change 34•50 cm to mm -
- move one place to the right = 345•0 mm.
Change 67•2 metres to Kilometres -
- move 3 places to the left = 0•0672 Km.

The mnemonic sentence/chart is one way to convert a measurement to smaller (or larger) units of the same length, but what you're actually doing mathematically is dividing (or multiplying) by 10 (or a power of 10, such as 1000 etc). Two more short mnemonics can help to summarise this process:

A separate page has a useful mnemonic for teaching young students about moving decimal places when multiplying or dividing.

Alternative mnemonics describing metric lengths are:

Kiss Henry Dirk’s Mother During Cow Milking.
King Henry Doesn't Mind Drinking Chocolate Milk.