locant
/ LOH-kant /
A locant is the number in a chemical name that tells you where on the parent chain something sits, like the house number in a street address. In 2-methylpentane, the '2' is a locant saying the methyl branch lives on the second carbon. Without locants you would know what pieces a molecule contains but not how they are arranged.
To assign locants, number the parent chain from one end to the other, then read off which carbon each substituent (and each functional group) is attached to. The rule is to number from whichever end gives the set of locants that is lowest at the first point of difference: you compare the two possible numbering directions term by term and keep the one whose first differing number is smaller. So a branch you could call 'carbon 4' from one end but 'carbon 2' from the other is reported as 2, because 2 beats 4.
Locants get glued into the name with hyphens (between number and word) and commas (between numbers), as in 2,3-dimethylbutane. When the lowest-locant rule still ties, you go to the next tiebreaker — for example, give the lowest locant to the substituent that comes first alphabetically. Locants are fussy bookkeeping, but they are exactly what makes a name unique instead of merely descriptive.
For a methyl on the third carbon of a six-carbon chain, numbering from the near end gives '3' and from the far end gives '4'; the name is 3-methylhexane because the locant set {3} beats {4}.
Number from the end that gives the lowest locant at the first point of difference.
The rule is 'lowest set of locants at first point of difference', not 'lowest sum'. Compare the ordered lists term by term; {2,2,5} beats {2,3,4} because 2 = 2 then 2 < 3.