Frame agreement
Frame agreement is a conformability rule which describes the conditions that must be satisfied by the frames of arguments to dyadic functions with rank (either derived and/or, when supported, native). The frames must either be identical, or one must be empty, or, more generally, when supported, one frame must be a prefix of the other.
Empty frame agreement
In SHARP APL and Dyalog, frames agree only if they match, or if one frame is empty. The latter case corresponds to pairing one argument in its entirety with each of the cells of the other argument.
Examples
x←⍳2 y←2 3 2⍴⍳12 x{⍺⍵}⍤99 2⊢y ⍝ 1ton pairing; ⍤99 corresponds to empty frame ┌───┬─────┐ │1 2│1 2 │ │ │3 4 │ │ │5 6 │ ├───┼─────┤ │1 2│ 7 8│ │ │ 9 10│ │ │11 12│ └───┴─────┘ x{⍺⍵}⍤0 2⊢y ⍝ 1to1 pairing; frames match; (,2) ≡ (,2) ┌─┬─────┐ │1│1 2 │ │ │3 4 │ │ │5 6 │ ├─┼─────┤ │2│ 7 8│ │ │ 9 10│ │ │11 12│ └─┴─────┘
Frame prefix agreement
In A+, BQN, and J, frames agree if one is a prefix of the other. In J, because every function has associated ranks, frame agreement generalizes leading axis agreement.
Description
A dyadic function with left and right ranks l and r splits its left argument into lcells, and splits its right argument into rcells. Each argument's shape is thus split into a frame and a cell shape. Given that one frame must be a prefix of the other, the shorter frame is called the common frame, which may be empty. Here, the generic term "cells" will denote the lcells (for the left argument) or rcells (for the right argument). If the frames are identical, the cells are paired 1to1 between the arguments. If the frame lengths differ, each cell of the shorterframed argument is paired with each among the corresponding group of cells of the longerframed argument. This 1ton pairing can be viewed as extending the shorter frame to match the longer frame. The collective results of the individual applications of the function are framed by the longer frame.
Examples
x=: i.2 y=: i.2 3 2 x+"0 1 y 0 1 2 3 4 5 7 8 9 10 11 12
The table below shows the pairing of cells from the above example. Here, the notation [cell shape]
denotes the cell shape, and 
denotes the division between the common frame and the remaining trailing axes.
Argument  Step 1: Frames / Cells  Step 2: Common frame—cells paired 

x 
2 [] 
2[]

y 
2 3 [2] 
23 [2]

The same example, but without considering cell shape:
Argument  Step 1: Frame / Cells  Step 2: Common frame 

x 
2 C 
2C

y 
2 3 C 
23 C

Based on the ranks 0 1
of the given function, x
's frame is ,2
and its cell shape is empty; y's frame is 2 3
and its cell shape is ,2
. The shorter of the frames, and thus the common frame, is ,2
. Relative to the common frame, each atom of x
is paired with the corresponding 3 vectors of y
.
The expanded example below uses APL to model frame prefix agreement.
⎕IO←0 ⍝ for comparison with J example x←⍳2 y←2 3 2⍴⍳12 l r←0 1 ⍝ the left and right ranks of the function +⍤0 1 ⊢lf rf←(l r)↓¨x,⍥(⊂∘⍴)y ⍝ frames ┌─┬───┐ │2│2 3│ └─┴───┘ ⊢cf←lf{⍺<⍥≢⍵:⍺ ⋄ ⍵}rf ⍝ common (i.e. shorter) frame 2 x{⍺⍵}⍤(≢cf)⊢y ⍝ 1st step: the (≢cf)cells are paired 1to1 between x and y ┌─┬─────┐ │0│0 1 │ │ │2 3 │ │ │4 5 │ ├─┼─────┤ │1│ 6 7│ │ │ 8 9│ │ │10 11│ └─┴─────┘ x{⍺⍵}⍤l r⍤(≢cf)⊢y ⍝ 2nd step: the (≢cf)cells in each pairing are split into l and rcells, and these are paired 1ton (or 1to1 if the frames are identical) ┌─┬─────┐ │0│0 1 │ ├─┼─────┤ │0│2 3 │ ├─┼─────┤ │0│4 5 │ └─┴─────┘ ┌─┬─────┐ │1│6 7 │ ├─┼─────┤ │1│8 9 │ ├─┼─────┤ │1│10 11│ └─┴─────┘ x+⍤0 1⊢y ⍝ ntom pairing; invalid in APL RANK ERROR x+⍤l r⍤(≢cf)⊢y ⍝ matches the result of J's frame agreement 0 1 2 3 4 5 7 8 9 10 11 12
Model
In dialects that do not feature frame prefix agreement, it can nevertheless be utilised by the introduction of an explicit operator:
_FA_←{ assert←{0≡⍵:'error: no common frame prefix' ⎕SIGNAL 4 ⋄ ⍵} r←1↓⌽3⍴⌽⍵⍵ ⋄ ar←≢¨p←⍴¨⍺⍵ ⍝ dyadic ranks, array ranks, shapes c←r{⍺>0:⍺⌊⍵ ⋄ 0⌈⍺+⍵}¨ar ⍝ cell ranks fl fr←(c)↓¨p ⍝ left and right frames s←fl{⍺<⍥≢⍵:⍺ ⋄ ⍵}fr ⍝ shorter frame k←{⍬≡⍵:99 ⋄ ≢⍵}s ⍝ relative rank assert ⍺∧⍥(s≡(≢s)↑⍴)⍵: ⍝ do frames agree? ⍺ ⍺⍺⍤c⍤k⊢⍵ } x+_FA_ 0 1⊢y 0 1 2 3 4 5 7 8 9 10 11 12
APL features [edit]  

Builtins  Primitives (functions, operators) ∙ Quad name 
Array model  Shape ∙ Rank ∙ Depth ∙ Bound ∙ Index (Indexing) ∙ Axis ∙ Ravel ∙ Ravel order ∙ Element ∙ Scalar ∙ Vector ∙ Matrix ∙ Simple scalar ∙ Simple array ∙ Nested array ∙ Cell ∙ Major cell ∙ Subarray ∙ Empty array ∙ Prototype 
Data types  Number (Boolean, Complex number) ∙ Character (String) ∙ Box ∙ Namespace ∙ Function array 
Concepts and paradigms  Conformability (Scalar extension, Leading axis agreement) ∙ Scalar function (Pervasion) ∙ Identity element ∙ Complex floor ∙ Array ordering (Total) ∙ Tacit programming (Function composition, Close composition) ∙ Glyph ∙ Leading axis theory ∙ Major cell search ∙ Firstclass function 
Errors  LIMIT ERROR ∙ RANK ERROR ∙ SYNTAX ERROR ∙ DOMAIN ERROR ∙ LENGTH ERROR ∙ INDEX ERROR ∙ VALUE ERROR ∙ EVOLUTION ERROR 