Two half-inch cassette formats reached households within a year of each other, and for most of a decade a buyer choosing a recorder was also choosing which shelf of the rental shop would be useful to them. The formats were not compatible: the cassettes were different sizes, the tape was threaded around the head drum along different paths, and a tape recorded on one machine would not play on the other.
The differences that decided the argument were not the ones usually argued about. Both formats laid the picture down in diagonal stripes across a half-inch tape using a spinning head drum, and both traded picture detail for running time in much the same way. What separated them in practice was how much programme a single cassette held at the price people were willing to pay for blank tape.
Running time as the deciding feature
A household buying blank tape wanted a film, a match or an evening's broadcast to fit on one cassette without changing it halfway. The larger shell and the slower standard speed of the format that won gave it that margin first, and once a longer tape existed the slower recording speeds extended it further, at a visible cost in picture quality. By the time the rival format matched the running time, the rental shelves had already been stocked on the assumption that most machines in the country were of one kind.
Picture and sound
Both formats recorded a composite picture with the colour information shifted down the frequency range and recorded underneath the luminance, which is why colour on tape looks softer and smears more readily than the outline of the image. Sound began as a single linear track along the edge of the tape, running at the same slow speed as the picture and sounding like it. The later addition of hi-fi audio tracks recorded by the spinning head drum, in the same diagonal stripes as the picture, produced a step in sound quality far larger than anything achieved for the picture.
What the incompatibility meant day to day
Because tapes could not cross between formats, ownership of a machine was a long commitment. Shops stocked what the majority of local machines could play; owners of the minority format found the new-release wall thinning first, then the catalogue behind it. That is the ordinary shape of a format decided by distribution rather than by measurement: the technical gap narrowed while the practical gap widened.
Where the formats agreed
It is worth saying plainly that both formats did the same remarkable thing. They let a household keep a broadcast, watch it later, watch it again, and hand it to somebody else. Nothing in the argument between them changed that, and almost everything about how home viewing worked afterwards followed from it.