Final answer:
The question pertains to the English spelling rule often referred to as Medical Rule 6, which guides how to pluralize words ending in YX, AX, or IX by changing 'X' to 'C' and adding 'ES'. It is related to the (y)-to-(i) rule, among others, in the spelling rules for suffixes.
Step-by-step explanation:
Spelling Rules for Suffixes in English
In English, certain spelling rules apply when adding suffixes to words. One such rule is Medical Rule 6, which relates to words ending in YX, AX, or IX. According to this rule, for the plural form, the 'X' is changed to 'C' and 'ES' is appended. Hence, 'appendix' becomes 'appendices', 'calyx' becomes 'calyces', and 'thorax' becomes 'thoraces'. This rule is similar to the (y)-to-(i) rule found in the 7: Spelling Rules for Suffixes chapter's section on replacing letters, which states that when a word ending in 'y' is pluralized, the 'y' is changed to 'i' before adding 'es'.
Other important rules from the same chapter include the Rule of Simple Addition, the Twinning or Doubling Rule, and the Final (e) Deletion Rule. For example, for the Final (e) Deletion Rule, when adding a suffix starting with a vowel, such as 'able' or 'ing', the final 'e' in a base word is dropped (hope + ing = hoping). These spelling rules are crucial for correct word formation in English and are particularly relevant in academic writing and communication.
Complete question:
Medical Rule 6" When a term ends in YX, AX, or IX, change the X to C and add ES
a-Appendix > appendices
b-Calyx > Calyces
c-Calix > callices (both are correct)
d-Thorax > thoraces