We need to stop saying the area of rectangles is 'length × width'.
There are so many ways we Maths teachers have made life difficult for ourselves by muddling up the language. Yes, we're trying our best to wean ourselves off saying 'minus', and specifying more clearly whether we're talking about subtraction or negative values. But we can do better than that. (And let's face it, the word 'division' isn't always exactly helpful either - are we talking about grouping or sharing?)
The area of rectangles is another example. We say 'length × width' for the very first area formula we teach, and then change the words we use for all the others just as they're getting their heads around the concept. And to start with, 'length' tends to imply 'the longer side', meaning 'width' must be 'the shorter side', but then width turns into 'the distance across from left to right' for all the other shapes that follow.
So let's start with rectangles being 'width × height', establish that we mean a vertical height at right angles to the width, and work from there with consistent language. And a few steps along, while we're at it, teach trapeziums as 'average width × height' rather than crossing our fingers and hoping they learn the formula.
This is how I now teach it (and I'll edit and add more resources to this post as I create them):
All Area Rules Summarised (not including circles)
This presentation is particularly suited for classes who've seen it all before and need a refresher, recap or revision. It reminds them of the basic principles they started with when they did rectangles, and includes rectangles, triangles, parallelograms and trapeziums, with one example of each.
All Area Rules Summarised (Including Circles)
This one is the same as above but also includes area of circles done by drawing a radius square first and then going on the principle that the circle is three and a bit times bigger than the square.
And below, presentations for each individual shape as I make them, starting with areas on grids...
Area of Straight-Edged Shapes on Square Grids
This as an editable powerpoint and three differentiated worksheets now available for sale on TES.