Rules are an important part of phonology. In certain cases it is claimed the rules need to be ordered. If the first rule creates an environment in which the second can apply, the rules are in a feeding relationship. However, if the first rule creates an environment in which the second can no longer apply, they are in a bleeding relationship.
For example, in many varieties of Brazilian Portuguese, the word "cidade" (city) is pronounced with a word-final [i] and the [d] is pronounced as a voiced alveopalatal affricate that corresponds to the English "j". The affricate only occurs before the high front [i]. To explain this sound change, two rules can be created. Rule A is unstressed word-final vowel raising which changes the mid front vowel to a high one. Rule B is palatalization which changes the voiced dental plosive to an affricate. The two rules are in a feeding relationship. The application of the first rule, vowel raising, creates the environment in which palatalization can occur.
However, this suggests that one rule precedes the other. It is possible that vowel raising and palatalization apply simultaneously. Though certain Brazilians apply word-final vowel raising but not palatalization, this does not prove that the two rules cannot apply at the same time. The concept of rule ordering does not necessarily have to exist in reality but simply on an abstract level.
A well-known example of feeding occurs in French. In the phrases "ma petite maison" (my small house) and "mon petit lit" (my small bed) the word for "small" is pronounced differently. The word "petite" has a word-final plosive but "petit" does not. It is claimed that there are two rules. One rule is word-final vowel deletion and the other is word-final consonant deletion. In "petite" the final vowel is deleted and in "petit" the word-final consonant is deleted. Rule A, word-final vowel deletion, feeds Rule B, word-final consonant deletion.
These rules are unsatisfactory on several levels. First of all, "petite" and "petit" are spelt differently. This is an indication that they should be pronounced differently. Second, "petite" is a feminine form which occurs with feminine nouns and "petit" is a masculine form which occurs with masculine ones. Phonology should not ignore spelling and grammar.
Another problem is that word-final vowel deletion is an optional rule. Though it is true that it is applied by most French speakers, in certain cases the vowel is pronounced as a schwa, particularly in careful speech. It does not apply categorically. In the phrase "mon petit lit", there is no evidence that a word-final vowel was deleted and then a word-final consonant. In fact, it is possible that the word-final vowel was added to created the feminine form.
In addition to feeding and bleeding, phonologists also use the terms counterfeeding and counterbleeding. These are terms used to describe cases in which the application of two rules is reversed.
In the phrase "mon petit lit" the two rules of word-final vowel deletion and word-final consonant deletion are applied to "petit" so that it is pronounced without a word-final schwa or word-final consonant. However, in the phrase "ma petite maison" the word-final vowel is deleted but not the word-final consonant. Many phonologists label this counterfeeding. They explain that the application of the rules is reversed. Rule B, consonant deletion, applies before Rule A, vowel deletion. If we apply Rule B first, it is blocked because "petite" has a word-final vowel. Then we apply Rule A, vowel deletion, to derive the correct pronunciation of "petite". Since one of the rules applies, this is labelled counterfeeding. The problem, however, is that there is no reason that "petite" should be pronounced the same as "petit". In fact, distinct masculine and feminine adjective forms are a characteristic of not only French but all Romance languages.
Counterbleeding can be clarified with an example from Portuguese. The word "bom" (good) has a fully nasalized vowel but no nasal. Two rules can be formed. Rule A is the vowel nasalization rule which states that a vowel becomes nasalized if it is followed by a nasal in the same syllable. Rule B is the nasal deletion rule which deletes a word-final nasal. If the order of the two rules were reversed and we applied nasal deletion first, the word "bom" would become "bo". In this case, the environment for vowel nasalization would be destroyed and it would fail to apply. This is an example of counterbleeding.
However, this example also has problems. It is not always the case that a vowel becomes nasalized before a nasal segment. For example, in Hungarian vowels do not nasalize before nasals. In the Hungarian word "szent" (saint) the "e" is not nasalized. Thus vowel nasalization is not a universal phenomenon. Second, it is not clear that vowel nasalization first applies and then nasal deletion. It is a possibility that the two rules apply simultaneously so that they coalesce. If the vowel and the nasal coalesce, neither vowel nasalization nor nasal deletion precedes the other.
Phonological rule orders often present many problems. One problem is that it is not always evident that one rule must precede the other. Other problems are that the rules are often not universal and in certain cases are optional rules. Phonological rules and rule orders often fail to capture the reality of the sound changes they represent.