-io → -i

In Italian, masculine nouns ending in -io follow two main paths when becoming plural:

• Unstressed -io (Atone): When the stress falls on a previous syllable, the plural ending -i replaces the -io. This applies to most regular masculine nouns with an unstressed -io, such as armadio (armadi), negozio (negozi), and esercizio (esercizi). This is very common in words ending in -cio, -gio, -glio, -chio, and -ghio (e.g., bacio becomes baci, figlio becomes figli, occhio becomes occhi).
• Stressed -io (Tonic): When the stress falls directly on the i (zì-o, invì-o), you keep that i and add the plural ending -i, resulting in -ii (zii, invii). Note that in most words ending in -gio, the 'i' is a silent diacritic used to make the 'g' soft. However, in leggìo (lectern), the 'i' is a stressed vowel, which is why it perfectly follows this rule and becomes leggii.

Ambiguity and Homographs: When two words are spelled the same but have different meanings (like prìncipi meaning 'princes' vs. princìpi meaning 'principles'), modern Italian uses a tonic accent to clarify the meaning if the context is not enough. You may also encounter a circumflex accent (î) in older literature (e.g., studî for the plural of studio).

Special Exceptions:

• Dio: The word for 'God' is unique. Its plural is dèi (gods) and it takes the article gli (gli dèi). The accent helps distinguish it from the preposition dei.
• Tempio: The word for 'temple' (tempio) usually becomes templi, reflecting its Latin root. While tempi is technically valid, templi is highly recommended to avoid confusion with the plural of tempo (time/weather).
• Nouns with Irregular Feminine Plurals: Certain collective nouns like paio (pair), centinaio (hundred), and migliaio (thousand) change gender in the plural, becoming feminine and ending in -a: paia, centinaia, migliaia. The unit of measurement miglio (mile) also follows this pattern, becoming le miglia (miles). Note the contrast with its homograph miglio (millet), which has a regular masculine plural, i migli.
• Body Parts: Several body parts ending in -io have irregular feminine plurals in -a, such as braccio (le braccia), ginocchio (le ginocchia), ciglio (le ciglia), and sopracciglio (le sopracciglia). Note that some of these have a regular masculine plural with a different meaning (e.g., i bracci refers to the arms of a machine or a cross).
• Feminine Nouns in -io: Feminine nouns ending in -io, such as la radio, are invariable in the plural (le radio).