Commonly Confused Suffixes: -able vs. -ible  

The suffixes “-able” and “-ible” are both used to form adjectives meaning “possible, capable of, suitable for, or causing.” Of the two, “-able” is much more common: it is what’s known as a “living” or “productive” suffix, meaning that it is still being used to create new words. The variant “-ible,” on the other hand, is only used in older words that have survived into modern English.

Because they are spelled so similarly and have the same pronunciation (/əbəl/), it can sometimes be hard to remember which is the correct one to use. Unfortunately, there are not very many conventions we can follow to know which suffix to use (and those that do exist are not always reliable). Instead, this section will focus on the different ways “-able” and “-ible” each attach to words, along with many examples for each.

The suffix “-able” most commonly attaches to words (most often verbs) that could otherwise be complete without the suffix; in many cases, this occurs with no change to their spelling. Because “-able” is a productive suffix, there is a huge number of words it can be attached to. Let’s just look at some common ones:

While many base words can take the “-able” suffix without changing their spelling, there are also many instances in which the spelling must be altered slightly in order for the suffix to be attached. The most common of these occur with words ending in silent E, but there are other instances in which a word’s spelling will change in various ways.

The most common spelling change made to a word when “-able” is attached is for silent E to be omitted from the end of the word and replaced with the suffix (though this is not always the case). For example:

Note that there are many instances in which we do not omit a silent E when adding the vowel suffix “-able.” This is especially true when it comes after C or G to make it clear that the consonants retain their “soft” pronunciations (/s/ and /ʒ/, respectively). For example:

(*In British English, the silent E is usually kept in the word ageing, whereas it is usually omitted in American English.)

While most common when coming after C/G + E, this convention of keeping E before “-able” does occur after other consonants as well. However, this is quite rare and, in many cases, is simply an alternative spelling (especially in American English, in which E is much more likely to be omitted).

Here are some examples in which you might see “-able” following a silent E:

When the last syllable of a verb contains a single short vowel followed by a single consonant, we usually have to double the consonant before “-able.”

For example:

Note that, in multi-syllable words, this is only the case if the final syllable is stressed in the base word. Otherwise, the final consonant is not doubled, as in answerable, cancelable, limitable, etc.

We already saw that many verbs ending in “-ate” will take the suffix “-able” by omitting silent E. However, there are also quite a few words in which “-ate” is replaced altogether. Unfortunately, there is no spelling pattern we can use to indicate which “-ate” words will be changed this way; we just have to memorize them. Here are some of the most common:

When a suffix beginning with a vowel is attached to a word ending in a consonant + Y, we almost always change Y to the letter I. Because of this, verbs that end in “-y” always take the “-able” suffix, since we never have a word spelled “-iible.” Let’s look at some of the most common examples:

There are also two nouns ending in “-y” that can take “-able,” but Y no longer changes to I—it is simply omitted:

Because “-able” is so commonly used with existing base words, the most common tip to remembering the “-ible” variation is that it is usually used with Latin roots that cannot stand alone as words.

However, this is not a very reliable convention, because “-able” also attaches to many incomplete Latin roots (though “-ible” does so more commonly). There is only one rule we can use to know which ending is correct: if a root ends in a “soft” C (/s/) or G (/ʒ/), it will always be followed by “-ible,” while a “hard” C (/k/) is always followed by “-able.” (No Latin roots will end in a “hard” G, /g/.) Unfortunately, we just have to memorize the rest of these types of words:

(*Note that the root fall- here is not the same as the verb fall.)

Though much less common than “-able,” the “-ible” variant can also attach to existing base words with no change to its spelling. As with the Latin roots, there are generally no indications in the base word’s spelling to indicate when “-ible” is correct, except for one: base words ending in “-uct” will (almost) always take “-ible” rather than “-able.”

Let’s look at some examples:

(Because “-able” is a productive suffix, meaning it is still being used to create new words, there may be instances in the future in which this rule is no longer true. For example, the term instructable has been gaining in popular usage in the last 30 years, but it is not found in the dictionary; instructible, meanwhile, is in the dictionary but has nearly become obsolete, which is why it isn’t included above.)

Like “-able,” the “-ible” ending can also replace silent E at the end of existing base words, as in:

However, as you can see by the size of the list, it is much less common for a silent E word to take “-ible” rather than “-able.” If a word ends in a silent E, it will most likely take the “-able” suffix (and base words ending in “-ate” can only take “-able”).

Sometimes an “-ible” word is related to an existing base word, but the spelling must change slightly to accommodate it. This also occurs with certain words when they attach to “-able” (e.g., when the suffix replaces “-ate” or “-y” becomes “-i-”), but “-ible” can result in much more drastic changes to the spelling of the base word.

The most consistent of these changes is for verbs ending in “-mit”: with the exception of limit (which becomes limitable), all of these verbs take the “-ible” ending, with “-mit” changing to “-missible.” Another common change occurs with verbs ending in “-nd,” which changes to “-nsible” (however, other verbs ending in “-nd” can take “-able” instead, so we can’t use this verb ending as a rule to determine the appropriate suffix). In addition to these, two other specific words have their endings change when attached to “-ible.” Let’s look at all the words that go through spelling changes with the “-ible” suffix:

In addition to the trends we’ve seen above, there are words ending in “-able” that can alternatively be spelled “-ible,” and vice versa. In most cases, these are simply less common variants; other times, the meaning of the word is very similar but subtly different, depending on the ending used.

The table below shows all words that have acceptable variant spellings. For each pair, the most common spelling is in bold; if one is much more common than the alternative, it will be bold and underlined. Finally, if the two spellings have slightly different meanings, they’ll be marked with an asterisk (*) and elaborated upon further on.

*These pairs of words have slightly different meanings, which we’ll look at next.