The unity and simultaneity of the three circuits is most evident when viewed from the perspective of the total social capital i.e. of Capital in General. But, at the same time, this total social capital itself is always divided into the functional forms of capital peculiar to each circuit. In other words, into money-capital, productive-capital and commodity-capital. That is so whether we view these three functional forms as operating independently in the shape of Bank Capital, Productive Capital and Merchant Capital, or not. As industrial capital, the capitalist has to have a portion of their capital, at any one time, in each of these functional forms.
“The next form in which the process presents itself is that of a succession of phases, so that the transition of capital into a new phase is made necessary by its departure from another. Every separate circuit has therefore one of the functional forms of capital for its point of departure and point of return.” (p 107)
“Various fractional parts of capital pass successively through the various stages and functional forms. Thanks to this every functional form passes simultaneously with the others through its own circuit, although always a different part of capital finds its expression in it. One part of capital, continually changing, continually reproduced, exists as a commodity-capital which is converted into money; another as money-capital which is converted into productive capital; and a third as productive capital which is transformed into commodity-capital. The continuous existence of all three forms is brought about by the circuit the aggregate capital describes in passing through precisely these three phases.” (p 107)
Consequently, viewed from the perspective of the total process, and the unity of the three circuits, rather than any one phase being an interruption of the other circuits, it is the condition for their continuity. It is in this continuity, and the succession of stages that it is at the same time simultaneous.
“Capital as a whole, then, exists simultaneously, spatially side by side, in its different phases. But every part passes constantly and successively from one phase, from one functional form, into the next and thus functions in all of them in turn. Its forms are hence fluid and their simultaneousness is brought about by their succession. Every form follows another and precedes it, so that the return of one capital part to a certain form is necessitated by the return of the other part to some other form. Every part describes continuously its own cycle, but it is always another part of capital which exists in this form, and these special cycles form only simultaneous and successive elements of the aggregate process.
The continuity — instead of the above-described interruption — of the aggregate process is achieved only in the unity of the three circuits. The aggregate social capital always has this continuity and its process always exhibits the unity of the three circuits.” (p 107)
Viewed from the perspective of 'many capitals' i.e. at the level of the individual firms or industries, this continual process, however, also displays discontinuity. The most obvious example is in agriculture, because of its seasonal nature. Crops of different types can only be planted at certain times of the year, and harvested at other times of the year. As well as there being several months between planting and harvesting, there will also be several months between harvesting and planting. In that case, the money received from the harvest C' – M', itself lies fallow until such time as it can be used to buy productive capital, M – C (MP + L), as well as a portion of productive-capital (the land, machines, possibly labour-power) itself lying fallow. For that reason, there is an incentive to try to use crop rotation, to ensure land is continually in cultivation.
But, similar variations apply to all capitals. Clothing manufacturers will have higher demand for their products at certain times of year than at others, and therefore need to have more employed as productive-capital during those times. Ice Cream sellers do most business in the Summer, and need to work longer and buy more ice cream, and spend more money running their vans during that time than in the Winter. Sports centres have peak periods when they have higher attendance and need to employ more staff than during off peak periods.
Finally, different types of commodity will themselves require different proportions of money, productive and commodity-capital.