Post your games!

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-once you transition into the mid game, what you want to buy immediately is mobility boots in order to increase the amount of global pressure you can put on. Since you're not to going to be CSing, mobos speed boost is always up, meaning you will be able to put down wards faster and roam more efficiently. mid game is all about map control and rotations, so naturally as a support, you want to optimize your ability/team's ability to do so.
-for melee supports such as leona, you want to play very aggressively as a ranged support until they hit a certain level, as you have the advantage of being ranged. For leona for example. you have the ability to q-harass her easily until around level 3, where her all-in potential greatly outweighs your ranged advantage + ability to poke. So, if you try to q her at lv 3 and stand a bit too close to her, she has the ability to jump on you with e->w->q combo and force an all in. Sona is not very strong in all-in scenarios pre-6, so you wil most likely lose the trade or die.
-swap out your yellow trinket for a red trinket when you get a sightstone, as sightstone will provide you with enough wards to diminish the need for a yellow trinket
-you should understand major spots to ward (i.e buffs and baron), and when to ward them. for example, if dragon spawns in 5 minutes, you don't want to spend time warding to control the area.
-when you buy items, think about how it will work with relation to your team and the enemy team. If your enemy team has lots of magic AoE damage, you want to opt towards a locket at your big mid-game item. If the enemy team has a strong ability to lock down and focus 1 target, you want to opt towards crucible. if your team is lacking in the disengage category, you want to opt for a talisman.
-same logic with items applies to summoners. exhaust is useless if you're facing a team with no-dive potential, since if you want to be in range to exhaust someone, you will have to walk up to them, which puts you at a heavy risk of taking unecceary damage. heal if there's nothing better to go, and ignite if no one else in your team has ignite and you're facing someone with heavy sustain like mundo, or if you want to simply dominate your lane.
-time summoners and ultimates in lane with respect to masteries and runes (i.e supports and ryze have 4.5 min flash cooldown, while everyone else usually have 5). knowing what abilities your enemy has is very important, as it tells you if they have the advantage on you in terms of CD's or not, which tells if you need to play safe or aggressively. it also tells your jungler how vulnerable your opponents are, which cues them if their odds of pulling off a successful gank is higher or not (an opponent with no flash is easier to gank than an opponent without one).