Since the loss of the noun inflection system of earlier Celtic, plurals have become unpredictable and can be formed in several ways: by adding a suffix to the end of the word (most commonly -au), as in tad "father" and tadau "fathers", through vowel mutation, as in bachgen "boy" and bechgyn "boys", or through a combination of the two, as in chwaer "sister" and chwiorydd "sisters". Other nouns take the singulative suffixes -yn (for masculine nouns) or -en (for feminine nouns). Most nouns which inflect according to this system designate objects that are frequently found in groups, for example adar "birds/flock of birds", aderyn "bird"; mefus "a bed of strawberries", mefusen "a strawberry"; plant "children", plentyn "a child"; and coed "forest", coeden "a tree".