A Question for an Expert

The title of this post was in the subject line of an email to me. 

The note reads:

Hi Robin,

After about 70 years of knitting, I didn't realize I wouldn't know the answer to this question until I came upon it.

If a pattern calls for size 4 mm needles and farther on asks you to use needles one size larger, are they asking you to use size 4.5 mm or size 5 mm?

My answer:

I use that scenario to explain why bands in patterns don't always work properly. You need to look at the pattern source. Even then you may not match your needle with the designers plan. U.S. needles are all whole numbers except for the 10 1/2. If the pattern was written with U.S. sizes and converted to metric one size up is 4.5. However not all the charts agree on an exact equivalent  for each size. Currently, more metric needles are being used for accuracy, so the designer could mean one size up on the metric chart. The same problems exist with the UK/Canadian system. The simple answer is that only the original designer or sample maker knows for sure what their one size up means if it is not listed in the materials section of the pattern.

How do you know for sure which is the best size....you already know what I'm going to say, by knitting a swatch.

Needle conversion chart here.