Mapped interval

This is a beginner page. It is written to allow new readers to learn about the basics of the topic easily.
The corresponding expert page for this topic is Tmonzos and tvals.

A mapped interval is an interval that has been mapped by a mapping matrix for a regular temperament.

For example, if we begin with an unmapped, JI interval [math]\frac{10}{9}[/math] with prime-count vector (or "monzo") [math]\textbf{i} =[/math] [1 -2 1⟩, the mapped interval ~[math]\frac{10}{9}[/math] under meantone temperament [⟨1 1 0] ⟨0 1 4]} would have generator-count vector (or "tmonzo") [math]\textbf{y} =[/math] [⟨1 1 0] ⟨0 1 4]}[1 -2 1⟩ = [-3 2}.

Note that we've notated the mapped interval with a tilde, ~[math]\frac{10}{9}[/math], to indicate that its size is now approximate.

Here are several mnemonics for the use of [math]\textbf{y}[/math] as the symbol for mapped intervals:

The letter "y" is linguistically similar to the letter "i", the obvious letter for (just) intervals.
Visually, a "Y" also looks like a diagram showing—from the top—wo just intervals getting mapped to the same size.
A 'y' also looks like a 'g', which is fitting because [math]\mathbf{y}[/math] is a generator-count vector, associated with the generator tuning map [math]𝒈[/math], in the sense that intervals are associated with (tempered-prime) tuning maps [math]𝒕[/math], or in other words, [math]𝒕\textbf{i} = 𝒈\textbf{y}[/math].

A "mapped interval" could also be called a "tempered interval", however, "tempered interval" is more ambiguous; "tempered interval" could also refer to a size resulting from mapping an interval by a tuning map for a temperament (in the same sense that "interval" is used to refer to a "(just) interval's (size)", or it could even refer to a projected interval such as the [0 0 1/4⟩ generator of quarter-comma meantone. Only "mapped interval" unambiguously refers to an interval that has been transformed only by the mapping matrix for a temperament.

Mapped interval

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