Yonex has been a player in the racquet sports industry for decades and has made strong inroads into the string market as well. Tour stars Nick Kyrgios, Frances Tiafoe, and Naomi Osaka have adopted Yonex Poly Tour Pro as their string of choice and the junior ranks seem to be following suite.
Luxilon and Babolat have dominated the string market with Big Banger ALU Power and RPM Blast respectively. Though at the 2018 Junior Australian Open, Yonex Poly Tour Pro tied Babolat RPM Blast for the most used string with 11.5% of the players. This is a great sign that Yonex has stepped up their quality of high-performance tennis string.
Yonex’ Poly Tour line includes 8 co-polyester offerings, they are ordered here from softest to hardest. Remember co-polyester strings are firmer, more control oriented strings. Players looking for softer more powerful options should check out the multi-filament section below.
Poly Tour Air is the softest polyester offering from Yonex. It has a soft HR-Elastomer added to the string for better shock reduction. Air provides good power while reducing the negative effect many polyesters can have on the arm. The string is designed to help players with slower swing speeds add spin, but maintain comfort. Poly Tour Air touts 20% less shock than conventional polyester strings. Available in 1.25mm 16L in blue .
Poly Tour Pro is the most popular string among Yonex pro players and is easily identifiable by its less than subtle Flash Yellow color. 
A touch firmer than Poly Tour Air, Pro is still an arm-friendly polyester with durable and comfortable qualities. Pro was designed to benefit all swing speeds and players seeking more spin, power, and comfort. Available in Graphite and Flash Yellow in 1.20mm 17G, 1.25mm 16L, and 1.30mm 16G. Tiafoe at right (May 27, 2018 – Source: Matthew Stockman/Getty Images Europe)
Poly Tour Fire has been made famous by two-time Grand Slam Champ Angelique Kerber. Fire ups the snap back and spin and is designed for players with faster racquet head speed who need more control. Silicone Oil Infused Filament (SIF) technology combines the mono-filament polyester with Silicone Oil to increase string movement and create more spin. Available in 1.20mm 17G, 1.25mm 16L, and 1.30mm 16G in Red.
Poly Tour Strike earned publicity when Denis Shapovalov and Caroline Garcia switched to it during 2017. They both had breakout seasons and helped generate buzz about the new string from Yonex. Shapovalov shown below courtesy of (Clive Brunskill/Getty Images)
Strike is all about control and tension maintenance for players that take consistently full swings at the ball. Available in 1.20mm 17G, 1.25mm 16L, and 1.30mm 16G in Iron Gray
Poly Tour Drive’s unique octagonal shape is designed to provide recreational and competitive players with easy access to spin. Drive is a touch firmer than Strike, and should help even slower swing speeds generate more spin. Available in 1.25mm 16L in Silver.
Poly Tour Tough provides an excellent value for chronic string breakers. Tough is long-lasting and durable and since the cost is low restringing won’t break the bank. Poly Tour Tough is a great option for tennis teams and is available in Black in 1.25mm 16L.
Poly Tour Spin has a small following on the professional circuit and has a noticeable pentagonal shape. Yonex uses Giga Spin Structure technology to up the ante with regard to spin generation. Poly Tour Spin provides 14% increased spin performance when compared to conventional string. Used on tour by up and comer Casper Ruud, Poly Tour Spin is available in 1.25mm 16L and 1.20mm 17G in Cobalt Blue.
Poly Tour Spin G represents Yonex’s firmest feeling string in their catalog. Spin G uses Silicone oil Infused Filament (SIF) technology and combines the mono-filament polyester with Silicone Oil to increase string movement and create more spin. Spin G has a slight pentagonal shape, in an effort to create more spin and Yonex promotes an 8% increase in spin when compared to conventional strings. Spin G is available in 1.25mm 16L in Dark Red and Orange.
Along with the updates to their Polyester line up, Yonex has revamped their Multi-filament string selection as well. These nylon strings will be stretchier and softer playing than the Poly Tour Line.
Rexis is Yonex’ most high end multi-filament string. It helps generate power and durability with the help of the FRF (Fusion of Resin Filament) production process. The string has greater elasticity and feels solid due to the gaps between the filaments being filled with a new flexible material called Fortimo. 
WTA star Naomi Osaka (right) plays with Yonex Poly Tour Pro in blended with Rexis for a unique blend of control and power. Rexis is available in 1.25mm 16L and 1.30mm 16G in off-white. (May 31, 2018 – Source: Clive Brunskill/Getty Images Europe)
Yonex Tour 850 Pro has a combination of comfort and crisp feel. It isn’t too soft or too firm and offers good durability for a multi-filament string. Players looking for a firmer multi-string to hybrid with a polyester will find a good fit. Tour Super 850 also plays well in a full bed and is available in white in 1.30mm 16G and (Pro) 1.32mm 16G.
Yonex added a new nylon (mono-filament) string in 2018 called Dynawire. Designed to be a highly playable synthetic gut, Dynawire offers great power and durability for the intermediate segment of players. The string has a winding structure with a thin metal film built in to provide a highly playable and springy synthetic gut option. Dynawire is available in 1.25mm 16L and 1.30 16G in white w/ silver.
Recently, Mark Gonzalez of The Yonex Stringing team hosted a seminar at Tennis Express:
