I was also taught the same thing in school around 40 years ago +1 it seems that vacuum is the odd word out when placed in a lineup with (for example) continuum, individuum, menstruum, and residuum What's the difference between at hand, on hand and in hand At hand seems to me as if you have something in reach On hand is if you have something in stock And in hand can be used as if you have.
Is it necessary to put an article before the word "vacuum" Considering their primary meanings, vacuum is used more often in a scientific context, in which case it means space completely or partially absent of any matter/air If a 'vacuum cleaner cleaner' is a machine for cleaning vacuum cleaners, then the person who cleans the vacuum cleaner cleaner would be a 'vacuum cleaner cleaner cleaner'. A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in practice Physicists often discuss ideal test results that would occur in a perfect vacuum, which they simply call vacuum or free space, and use the term partial vacuum to refer to real vacuum. In other words, don't talk about how the vacuum cleaner was made in germany and incorporates the latest technology
