The DC-I (Double Cassette International) was published by Grundig in 1965. It is mechanically very similar to the Compact Cassette.
The cassette measures 120 x 77 x 12 mm. The tape width is the same as in the Compact Cassette, 3,81mm. The tape speed is a bit higher, 5,08 cm/s.
There were two lengths of tapes, C90 and C120. Telefunken's own record company Teldec also released pre-recorded cassettes.
When Philips made their Compact Cassette license free, all manufacturers started to build Compact Cassette recorders.
The DC-I cassette had actually just one thing that was "better" than the Compact cassette. It provided a recording time of two hours. However, a couple of years later also 120 minute Compact Cassettes came into the market. Grundig discontinued the DC-I cassette in the end of the 1960's.
DC-I -recorders were manufactured by Grundig, Telefunken and Blaupunkt. All DC-I recorders and players were mono devices. The format didn't live long enough to get to stereo.