Lokál singuláru tvrdých neživotných maskulin (ve vlaku vs. v potoce): úzus a gramatičnost

Approximately 6000 inanimate appellative masculine nouns in the locative singular case are used in contemporary written Czech. About 400 of these nouns use both the -u and the -ě/-e ending. In about 200 nouns the two variant endings occur in a frequency equilibrium or the historical primary -ě/-e prevails. The nouns which end with the -h,-g,-f consonants use only the -u ending without exception. The nouns that end with -k, -ch, -r, i.e. the consonants that alternate with -č, -š, -ř, and also the nouns ending with -p, -b, -m, which do not alternate, use only the -u ending as a norm, with only a very few exceptions found in standard written Czech. If the frequency and the historical progress of the -u ending are considered, the -ě/-e ending in some exceptional uses in nouns ending in -k, -ch, -r; -p, -b, -m can be regarded ungrammatical. The grammatical -ě/-e ending is used alternatively, or in rare cases, exclusively, with a considerable number of those nouns which end with -d, -t, -n, but mainly with -s, -z, -l.