A word-by-word translation of the German term Schräge Musik, would be slanted music, but it actually means jazz. It is more known, however, as the name for an arrangement of upward-firing guns, installed in German nightfighters and used with great effect against British night bombers. There was some irony in the choice of this name, because jazz was officially forbidden in Nazi Germany, rejected as a "degenerate" art.
There was nothing new about upward-firing guns. They first appeared during WWI, because the dominant type of aircraft, for all roles except that of fighter and long-range bomber, was a two-seat biplane with a tractor engine. In its more developed form the observer-gunner sat behind the pilot, armed with a flexible Lewis or Parabellum machinegun. He could defend the aircraft against attacks from the upper hemisphere, but was powerless against attacks from the rear and below. This could be exploited by interceptors, but an attack from below either required a climbing attack, which could not be maintained for long, or some form of upward-firing armament.
Left, a Lewis gun on a Foster mount. The aircraft is an SE.5a. Note the two posts for a ring-and-bead gunsight on top of the gun, used to fire it upwards. 
Before the introduction of gun synchronisation mechanisms some single-seat biplanes had been equipped with a fixed machinegun, usually a Lewis, on the center section of the upper wing, so that it could shoot over the propellor. The best known of these fighters was the Nieuport 11 Bébé. To allow the pilot to change ammunition drums, the Foster mount was developed: A curved rail, that made it possible to slide the gun backwards and downwards. In the latter position, the gun pointed upwards, and could be used for attacks from below. This was its only real advantage. Disadvantages were many: It caused drag, changing the ammunition drums was very difficult, and its recoil disturbed the aim. Nevertheless, the success that pilots such as Albert Ball had with the Foster mount explains its presence on the later Royal Aircraft Factory SE.5a, at a time when most fighter aircraft had two synchronized guns.
Some German pilots experimented with Foster mounts that had been captured. At the time, it was not unusual for pilots to order considerable modifications to their aircraft. But this seems to have had no influence on later German practice.
The Lewis guns on the upper wing also appeared on the Sopwith Dolphin and, interestingly, on Sopwith Camel fighters modified for nightfighting. In the latter case, however, the main purpose was to improve the forward view of the pilot by removing the Vickers guns in front of him. After the end of WWI most fighters had twin synchronized guns, even nightfighters.
Left, the Vickers model 161 was built to specification F.29/27. It was an obvious dead end. The prototype flew little and reportedly fired only 24 shells. 
During the interbellum upward-firing guns were tried on a number of British aircraft. This was called the no-allowance method of gunsighting, because the idea was that the attacker would fly in close formation with the target, slightly below it, so that no allowance had to be made for relative speed. Specification F.29/27 called for a single-engined, single-seat fighter armed with a 1½lb COW gun, angled upwards. Development of this weapon, by the Coventry Ordance Works, had continued since before WWI. At 97kg, it was relatively light for a weapon of this size. It fired 37x190 ammunition at a rate of 1.5 per second, fed from five-round clips, and had already been used on a handful of aircraft during WWI. Unfortunately, the two F.29/27 fighters were both disappointing. The Vickers F.29/27 was a pusher biplane, an obsolescent design with a bewildering array of struts and bracing wires. The Westland F.29/27 was a low-wing monoplane and looked much better, but had completely unacceptable handling characteristics.
More promising were experiments with the standard .303 Vickers and Lewis guns. In 1927 the Boulton-Paul Bittern made its first flight, a nightfighter built to specification F.27/24. The second prototype of the Bittern had vertically swivelling Lewis guns machineguns on the side of its nose, so that they could be set at an angle between 0 and 45 degrees. A ring-and-bead gunsight was to be mounted on a frame, which could be set at the same angle. But the underpowered Bittern never entered service.
A simpler approach was represented by two Bristol Bulldog biplane fighters, that were modified in 1934 with Lewis or Vickers guns mounted at the side of the cockpit, at an angle of 60 degrees up. During tests, the installation demonstrated great accuracy: Flying 100ft below their targets, the fighters scored 90% hits. However, the armament of two rifle-calibre machineguns was too weak.
One interesting aircraft, flown just before the beginning of WWII, must also be mentioned. The prototype of the Gloster F.9/37 twin-engined monoplane fighter was armed with five 20mm Hispano cannon, angled up 12 degrees. This angle was determined by practical considerations: Three of the guns were behind the cockpit, in the space originally allocated for a gun turret. They had to be angled up to clear the cockpit. It is not clear what attack mode was envisaged for this aircraft.
One of the first effective British nightfighters was the Boulton-Paul Defiant. The Defiant, armed with four .303 guns in a powered turret, had failed as a day fighter. Its natural approach as a nightfighter was to attack from below, because the guns in the turret could not fire straight forward. But the career of the Defiant as a nightfighter was fairly short, and apparently it made no impression on the Germans. Effectiveness was certainly limited, because of the light firepower and the blinding muzzle flash.
Shortly before the war, the idea of upward-firing guns for nightfighters had reached the Luftwaffe from several sources. A Leutnant Tiede, who had used upward-firing guns in WWI, approached the Reichsluftfahrtsministerium with his design, but it was rejected. Reports of Japanese experiments conducted in 1938 and 1939 were received, but apparently these too failed to make an impression. After the outbreak of war, there were several incidents in which observer guns were aimed at the belly of British bombers, but usually these were only 7.92mm weapons.
Apparently the instigator of the adoption of Schräge Musik by the Luftwaffe was Oberleutnant Rudolf Schönert, who started advocating this in 1941. The first installation was made late in 1942, in a Do 17Z-10 that was also equipped with Lichtenstein radar. The results were inconclusive, and development was shelved for a year. Nevertheless it is reported that in the summer of 1942 Schönert, then commanding II/NJG 5, received three Do 217J nightfighters for operational testing of this form of armament. (Schönert had his first combat success with Schräge Musik in May 1943, and then not in a Do 217J but in a field-modified Bf 110.) Wide-scale adoption followed in late 1943, and in 1944 a third of all German nightfighters carried upward-firing guns.
Above and left. One of the most effective German nightfighters was the Ju 88. This is a Ju 88G-6 with liquid-cooled Jumo 213 engines, Lichtenstein SN radar, and two Schräge Musik MG 151/20 cannon installed in the aft fuselage. 
There was more to Schräge Musik than just fitting a few angled-up cannon, usually MG 151/20 or MK 108. These were put in the rear of the cockpit of the Bf 110, in the aft fuselage of the He 219, and behind the cockpit of the Ju 88 and Do 217. It was important to attack undetected, and therefore tracers were not used. Special ammunition with a faint glowing trail replaced them. The guns were given flash reducers. An additional gunsight was installed in the cockpit to aim the guns. The attack from below had the advantage that the nightfighter crew could observe and identify the silhouette of the aircraft before they attacked. At the same time the bomber crew could not see the nightfighter against the dark ground, nor defend itself: The belly turrets of British bombers had been removed because of their limited effectiveness and to reduce drag. The nightfighter usually aimed for the fuel tanks, not for the fuselage, because of the risk that exploding bombs would damage the attacker. Schräge Musik soon produced devastating results. It was at its most successful in the winter of 1943-1944. This was a time when losses became unacceptable: The RAF lost 78 of 823 the bombers that attacked Leipzig on 19 February, and 107 of the 795 bombers that attacked Berlin on 30 March.
RAF Bomber command compensated for the German lateness to adopt this form of armament by reacting slowly to it. Reports of bomber crews gave no indication, because the German nightfighters managed to stalk their preys without being perceived. Only an analysis of the damage done to returning bombers demonstrated that the Germans were firing from below. This seems to have been understood fairly quick, for the problem was already reported in April 1943. However, it took considerable time to implement a satisfactory solution. Initially, a downward observation window was provided, and Canadian bombers again received belly turrets. But the effectiveness of these measures was small, because the attackers were very hard to see. Radar was a better solution, but the Monica tail-warning radar provided warning only if the attacker approached from astern, not from below. Anyway, in July 1944 the British discovered FuG 227 Flensburg in a captured German aircraft, a receiver that could be used to home in very accurately on the emissions of the Monica radar from a distance of 80km. The tail-warning radar then had to be deleted.
The H2S navigation radar, that had replaced the belly turret on many bombers, did look downwards; but it did not provide any warning of enemy aircraft approaching from below. The H2S display showed the radar image starting from the first ground return, so that a map could be drawn. Any echoes preceding this ground return were discarded -- The echoes of aircraft below the bomber. A modification of a H2S radar proceeded as soon as the developers became aware of the problem, and to avoid administrative delay, development was undertaken more or less clandestinely. In July 1943 the Fishpond modification of H2S was ready. A display screen was added, that indicated range and bearing of any aircraft below the bomber; an estimate of the relative height could be made by banking the bomber. But Bomber Command was large, and it took considerable time to install the new equipment. By the spring of 1944 most bombers carried Fishpond, and losses dropped sharply. However, there was considerable turmoil when it was discovered that German nightfighters carried the Naxos detector, that allowed them to determine the origin of H2S emissions. Only after the interrogation of prisoners made clear that Naxos was far too inaccurate to allow nightfighters to home in on an individual bomber, and at best gave an indication of the position of the bomber stream, was confidence in H2S and Fishpond restored.
An outgrowth of the Schräge Musik concept was the development of a number of vertically firing Sondergeräte, a term which can be translated as "special devices". These consisted of a number of recoilless single-shot guns, firing 30mm or 55mm ammunition. They were triggered by photo-sensitive cell. In theory, all the pilot had to do was pass at a suitable distance (100m to 50m) under a bomber. Use of these weapons remained experimental, and after the war nobody continued the concept.
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