# Flought scale

A scale is (k-)flought (/flɔːt/, rhymes with bought) if it is made of k > 1 copies (called strands) of an n-note periodic scale s, and any two copies of s are interleaved so that any note of the first copy falls strictly between two notes of the other copy. The set of offsets that separate the strands from a fixed strand is a chord called the polyoffset, which is determined up to inversion and equave-equivalence. A flought scale is thus a cross-set with a little additional structure. One can floughten or flought a scale s with a certain polyoffset Δ (or: "Δ floughtens s" or "s is floughtenable with Δ") if s is the strand scale of a flought scale with polyoffset Δ. Such a scale is denoted Flought(s; Δ). The concept of flought scales is a generalization of bipentatonic scales and (even-length) generator-offset scales.

Blackdye, Zil[14], and bicycle are examples of flought scales, because they each have two interleaved strands, respectively Pyth[5], Zarlino, and 8:9:10:11:13:14. The terminology, however, is intended to cover any number of strands and any choice of strand scale.

The term flought was coined by Inthar by evolving the Old English past participle (ġe)flohten of the verb fleohtan 'to weave; to plait' into a hypothetical native Modern English cognate to the words plait and plexus.

## Some flought scales

Flought scales can easily be built from a harmonic series mode as the strand: for example, if n::2n is the strand, then (2n + 1)/2n always works as the offset (e.g. strand 5:6:7:8:9:10, offset 10:11). Here are some other examples:

• Flought(12:14:16:18:21:24; 11:12)
• Flought(12:14:16:18:21:24; 12:13:22)
• Flought(12:14:16:18:21:24; 8:10:11)
• Flought(12:14:16:18:21:24; 9:10:11)
• Note: detempered 11-limit Porcupine[15]; well-formed generator sequence GS(10/9, 11/10, 12/11, 10/9, 11/10, 12/11, 10/9, 11/10, 189/176)
• Flought(Pyth[5]; 8:10:11)
• Flought(Pyth[5]; 9:10:11)
• Note: detempered 2.3.5.11 Porcupine[15]; well-formed generator sequence GS(10/9, 11/10, 12/11)
• Flought(9/8-14/11-4/3-3/2-56/33-21/11-2/1; 9/7)

## Properties

1. The following is a necessary and sufficient condition for floughtenability. Let S be a scale with equave E, $\mathcal{D}_k(S)$ be the set of all k-step dyads of S, and Δ be a chord such that every dyad of Δ falls within the open interval (0, E). Then the polyoffset chord Δ floughtens S if and only if no nonunison (positive) dyad in Δ falls within $[\min \mathcal{D}_k(S), \max \mathcal{D}_k(S)]$ for any k ∈ {0, ... len(S) - 1}.
2. For any periodic scale S with equave E, if δ is an offset and Flought(S; δ) exists, then Flought(S; δ) = Flought(S; E - δ) = Flought(S; δ + E). Thus, taking the equave complement of an offset in a polyoffset does not change the flought scale, nor does shifting any individual offset by equaves.
3. Given an E-equivalent scale S, offsets δ within the open interval (0, min({step sizes in S})) are called small in the context of floughtening S. Small offsets are significant because the resulting flought scale has a structure that closely mimics the underlying scale structure: if S is a circular word $w(a_1, a_2, ..., a_n)$ then Flought(s; δ) uses the same circular word but with δ followed by the difference between δ and every step size in w, namely $w(\delta b_1, \delta b_2, ..., \delta b_n)$ where $b_i = a_i - \delta$.
4. A flought scale is not always CS, even when the strand is CS and the scale has a generator sequence where every generator subtends the same number of steps. One such scale is Flought(Zarlino; 32/25) = 25/24 9/8 75/64 5/4 125/96 4/3 375/256 3/2 25/16 5/3 225/128 15/8 125/64 2/1 which has GS(32/25 125/96 32/25 5/4).
Proof of the offset constraints
If the polyoffset has more than two notes, the interleaving condition only needs to hold for pairs of distinct strands, and hence the above property only needs to hold for pairs of notes in the polyoffset. This reduces the proof to the case of one offset δ.

Let S1, S2 denote the two copies of S separated by δ, where S1(0) = 0 (the unison), S2(0) = δ. Assume that the scale F is the union of S1 and S2, and F(0) = 0. Let $m_k = \min \mathcal{D}_k(S)$ and $M_k = \max \mathcal{D}_k(S).$

Suppose δ > 0 is not in any intervals [mk, Mk], 1 ≤ kn − 1, n = len(S). Then for any k, S1(k) falls between adjacent notes of S2. The same holds when we reverse the roles of S1 and S2 and use the offset E − δ; since the union $\bigcup_{k=1}^{n-1} [m_k, M_k]$ is invariant under taking equave complements, neither is E − δ within any [mk, Mk]. The reverse implication follows.

For the forward implication, we wish to show that the interleaving condition is violated if mk < Mk and δ ∈ [mk, Mk] for some k, 1 ≤ kn − 1. We first observe that if mk < Mk, then S has some pair of stacked k-steps, say (S(n0), S(n0 + k)) (S(n0 + k), S(n0 + 2k)), whose sizes t0, t1 are unequal and both contained in [mk, Mk]. Moreover, such intervals [t0, t1] or [t1, t0], taken over all non-edE circles of k-steps in S, must cover [mk, Mk]. Indeed, if a circle of stacked k-steps in S has the k-step Mk, that circle must also have a k-step smaller than k/gcd(n, k) steps of n/gcd(n, k)-edE, and by symmetry, the previous clause also holds when "Mk" and "smaller" are replaced with "mk" and "larger".

The covering of [mk, Mk] constructed above grants us a stacked pair t0, t1 of unequal k-steps in S such that δ ∈ [t0, t1] ⊆ [mk, Mk]. Assume t0 < t1. (If t0 > t1, take equave complements and use the offset E − δ.) Then the corresponding occurrence of the k-step t0 in S2 is shifted into the closed interval I corresponding to the k-step t1 in S1. But we then have k + 1 notes of S2 within I. Assuming none of these notes coincide with a note of S1 (otherwise, interleaving would be violated), each of the k + 1 notes must fall within one of the k scale steps subtended by t0 in S1. By the pigeonhole principle, at least one of these steps in S1 must contain two consecutive notes of S2 in its interior, breaking the interleaving condition as desired. $\square$

## Generalizations

### Co-floughtenability (?)

Periodic scales $S, T : \mathbb{Z} \to \mathbb{R}$ of the same length and equave are co-floughtenable (the co- is not meant to suggest any kind of duality) if there exists $\delta\in\mathbb{R}$ such that S and T + δ are interleaved. Note that though a given 2n-note scale being a co-floughtened result of some pair of scales may be trivial, a given pair of scales being co-floughtenable is less so: for example, MMMM and Lsss are not co-floughtenable when s is too small.

A contraflought scale is a co-floughtened pair of the two chiralities of a chiral scale.