The following cmavo are discussed in this section:

Sometimes the place structures engineered into Lojban are inadequate to meet the needs of actual speech. Consider the gismu viska, whose place structure is:

Seeing is a threefold relationship, involving an agent (le viska), an object of sight (le se viska), and an environment that makes seeing possible (le te viska). Seeing is done with one or more eyes, of course; in general, the eyes belong to the entity in the x1 place.

Suppose, however, that you are blind in one eye and are talking to someone who doesn't know that. You might want to say, “I see you with the left eye.” There is no place in the place structure of viska such as “with eye x4” or the like. Lojban allows you to solve the problem by adding a new place, changing the relationship:

The three-place relation viska has now acquired a fourth place specifying the eye used for seeing. The combination of the cmavo fi'o (of selma'o FIhO) followed by a selbri, in this case the gismu kanla, forms a tag which is prefixed to the sumti filling the new place, namely le zunle. The semantics of fi'o kanla le zunle is that le zunle fills the x1 place of kanla, whose place structure is

Thus le zunle is an eye. The x2 place of kanla is unspecified and must be inferred from the context. It is important to remember that even though le zunle is placed following fi'o kanla, semantically it belongs in the x1 place of kanla. The selbri may be terminated with fe'u (of selma'o FEhU), an elidable terminator which is rarely required unless a non-logical connective follows the tag (omitting fe'u in that case would make the connective affect the selbri).

The term for such an added place is a “modal place”, as distinguished from the regular numbered places. (This use of the word “modal” is specific to the Loglan Project, and does not agree with the standard uses in either logic or linguistics, but is now too entrenched to change easily.) The fi'o construction marking a modal place is called a “modal tag”, and the sumti which follows it a “modal sumti”; the purely Lojban terms sumti tcita and seltcita sumti, respectively, are also commonly used. Modal sumti may be placed anywhere within the bridi, in any order; they have no effect whatever on the rules for assigning unmarked bridi to numbered places, and they may not be marked with FA cmavo.

Consider Example 9.26 again. Another way to view the situation is to consider the speaker's left eye as a tool, a tool for seeing. The relevant selbri then becomes pilno, whose place structure is

and we can rewrite Example 9.26 as

Here the selbri belonging to the modal is se pilno. The conversion of pilno is necessary in order to get the “tool” place into x1, since only x1 can be the modal sumti. The “tool user” place is the x2 of se pilno (because it is the x1 of pilno) and remains unspecified. The tag fi'o pilno would mean “with tool user”, leaving the tool unspecified.