Concerning the numbering of locants for substituents, the most important simplified criteria are:

 1. lower locants for the principal characteristic group that is expressed as suffix
 2. lower locants for prefixes
 3. lower locants for substituents cited first as a prefix in the name

The corresponding actual wording in the current version of *[Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)](http://dx.doi.org/10.1039/9781849733069)* reads as follows:

> **P-14.4** NUMBERING
>
> When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
>
> (…)
>
> (c) principal characteristic groups and free valences (suffixes);
>
> (…)
>
> (f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
>
> (g) lowest locants for the substituent cited first as a prefix in the name;
>
> (…)


The $\ce{-OH}$ group is the principal characteristic group in this case. It is expressed as suffix ‘ol’.

Therefore, low locants are assigned first to the $\ce{-OH}$ groups according to Rule (c), which yields ethane-1,1-diol.

After that, a low locant is assigned to the chloro groups according to Rule (f); however, the only possible numbering left is **2,2,2-trichloroethane-1,1-diol.**