Our Transitional elbow will be offset from one side only. Here you will layout a 90 Deg. elbow 8/12 to 8/10 keeping one side straight. Draw your flat side pattern and bisect the heel and throat curves, establishing points 3 & 6.
Here is how we bisect the throat to find point 6. What is the reason for establishing points 3 & 6? We know in the flat pattern where our radius pivot point is, however when we layout the pattern for the offset cheek we need to establish the pivot point for the heel and the throat. We will be making the offset much like three hinged triangles. We need to divide the side up so we have the bending points in the right place.  
Here is how you can bisect the heel, we show here the heel and the throat being done at the same time. If this were a transitional elbow with different widths we would not be able to do it this way. Now establish point 3. 

Once we have established all our points. From these points we determine our true lengths and transfer them to the True Length bar as we did in Fig. 5 Remembering A being your horizontal plane surface and B being the distance of your offset. C equals half the distance of the offset. On transitional elbows the distance AC is made only half the full distance AB, Why? well if you look at Fig.12 you see that only half the height is stepped down from 1 to 3 and 2 to 3, the second half is stepped down from 3 to 4 and 3 to 5, Take trammel points and transfer the distance of 2 to 3, 1 to 3, 2 to 4 and 2 to 6 from the flat cheek pattern to your true length bar, number each point (reference from later) and draw a straight line to point C and B, in Fig. 5 below. 

NEXT WE BEGIN TO DEVELOP THE DROP CHEEK.  
Start by drawing your horizontal plane surface 1 to 2. Using 2 as a radius point and the distance of (C to 23) of your true length bar make a reference mark (slight arc) near point 3 in (Fig 6). Now take the distance of (C to 13) of your true length as a radius and point 1 in Fig. 6 as a center make a reference mark crossing the one previous drawn. Now using the distance of (B to 24) as a radius from your true length bar and 2 as a center make a reference mark near point 4 in Fig. 6. Now with point 3 as a center, and point 23 as a radius draw a mark that crosses near point 4. Now remembering back in Fig 1, we bisected the heel to establish point 3, and knowing this is the center we can now take the distance of point 1 to 3 and using 3 as a center make a reference mark (slight arc) near point 5. your distance from 45 is your height of your duct. Using this measurement and setting your trammel points establish point 5 in Fig. 6 

We need to establish point 6 on the cheek with the offset. To do this you need to take from your true length bar the distance of (C to 26 ) as your radius in Fig 5 and point 2 in Fig 7 as a center point, and draw a reference mark near point 6, keeping the radius the same and using point 4 as a center draw another reference mark that crosses the previous mark near point 6. Drawing your arc through points 2,6 and 4 you need to bisect 26 and 64 as we did here in Fig. 7 to establish P1. P1 will be the center for the arc. (Use the same point P1 and set your trammel points back to the allowance you need for your connection. 

Pictured is the arc drawn from the center point P1  
Here we added 1" on each opening for the connectors ( sslips & drives.) It really isn't necessary to draw the lines from 2 to 3 and 3 to 4, as long as you know where the brake points are. We suggest a mark on the inside of the fitting so it is broke the correct way. 

LAYING OUT THE THROAT AND HEEL Laying out the heel and throat is pretty basic, however you must take the length of your flat cheek pattern and not the length of the offset pattern. (A common mistake) 

Here in Fig.13 you can see where we get C from. A  B is the offset distance and C being the center, follow C to the right and you can see Point 3 which is centered with the side. 

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