The difference from English compounds is that German compounds are always written together as a single word: "spy satellite" is thus "Spionagesatellit" and "mad cow syndrome" "Rinderwahnsinn". Compound nouns take the gender of the last component noun (the head). In addition, there is the grammatical feature of the Fugen-"s": certain compounds introduce an "s" between the noun stems, historically marking the genitive case of the first noun (cf. Idafa), but it occurs frequently after nouns which do not actually take an "s" in their genitive cases. In many instances, the compound is acceptable both with and without the "s", but there are many cases where the "s" is mandatory and this cannot be deduced from grammatical rules, e.g. "Hochzeitskleid" = "wedding dress", "Liebeslied" = "love song", "Abfahrtszeit" = "time of departure", "Arbeitsamt" = "employment agency". Occurrence of the Fugen-"s" seems to be correlated to certain suffixes (of the first stem); compounds with words in "-tum, -ling, -ion, -tät, -heit, -keit, -schaft, -sicht, -ung" and nominalized infinitives in "-en" mostly do take the "s", while feminine words not ending in "-ion, -tät, -heit, -keit, -schaft, -sicht, -ung" mostly do not, but there are exceptions. Use of the "s" is mostly optional in compounds in which the second element is a participle. To reduce length or to highlight distinctions, a prefix or suffix is sometimes mentioned only once but applies to more than one compound noun. For example:  As in English, some nouns (e.g. mass nouns) only have a singular form ("singularia tantum"); other nouns only have a plural form (""pluralia tantum""):  Traps abound in both directions here: common mass nouns in English are not mass nouns in German, and vice versa:  Again as in English, some words change their meaning when changing their number:  A few words have two different plurals with distinct meanings. For example:  Some words share the singular and can only be distinguished by their gender and sometimes their plural: